Hydrate desalination for water purification

ABSTRACT

An apparatus is disclosed which allows the hydrate formed in the hydrate formation region of a desalination fractionation apparatus to be cooled as it rises in the apparatus. This has the beneficial effect of increasing its stability at lower pressures and reducing the depth at which the hydrate will begin to dissociate. The present invention provides for more efficient management of the distribution of thermal energy within the apparatus as a whole by controlling the flow of water through the system—particularly residual fluids remaining after hydrate forms—such that it is substantially downward through the fractionation column and out through a lower portion thereof. Hydrate thus separates from the residual fluid at or nearly at the point of formation, which helps keep the hydrate formation region of the apparatus at a temperature suitable for the formation of hydrate and improves efficiency. Hydrate formation may be enhanced, thereby further improving efficiency, by pre-treating the water-to-be-treated so as to dissolve hydrate-forming gas in it, before further hydrate-forming gas is injected into the water-to-be-treated under conditions conducive to the formation of gas hydrate.

This application claims priority from U.S. provisional patentApplication Ser. No. 60/230,790, filed Sep. 7, 2000. That provisionalpatent Application (and, accordingly, this Application) disclosesmethodology and apparatus for significantly improving the efficiency ofdesalination using gas hydrates as taught in Applicant's co-pending U.S.patent application Ser. No. 09/500,422 (“the '422 application”), filedFeb. 9, 2000, and its precursors. The subject matter of the '422application was incorporated in provisional patent Application Ser. No.60/230,790 by reference; that subject matter is recited expresslyherein.

This application also claims priority from U.S. provisional patentApplication Ser. No. 60/240,986, filed Oct. 18, 2000.

The '422 application is a continuation-in-part of co-pending applicationSer. No. 09/397,500, filed Sep. 17, 1999, which is acontinuation-in-part of co-pending application Ser. No. 09/375,410,filed Aug. 17, 1999, which is a continuation-in-part of co-pendingapplication Ser. No. 09/350,906, filed Jul. 12, 1999.

FIELD OF THE INVENTION

The present invention generally relates to desalination or otherpurification of water using gas hydrates to extract fresh water fromsaline or polluted water. In particular, the invention relates todirecting the water flow and managing the movement of hydrate to obtainmaximum efficient heating and cooling of the water-to-be-treated eitherby heat absorption or dilution using cool fluids or other cooling in thearea of hydrate formation.

BACKGROUND OF THE INVENTION

In general, desalination and purification of saline or polluted waterusing buoyant gas hydrates is known in the art. For example, U.S. Pat.No. 5,873,262 discloses a water desalination or purification methodwherein a gas or mixture of gases spontaneously forms buoyant gashydrate when mixed with water at sufficiently high depth-inducedpressures and/or sufficiently low temperatures in a treatment column.According to prior technology, the treatment column is located at sea.Because the hydrate is positively buoyant, it rises through the columninto warmer water and lower pressures. As the hydrate rises, it becomesunstable and dissociates into pure water and the positively buoyanthydrate-forming gas or gas mixture. The purified water is then extractedand the gas is processed and reused for subsequent cycles of hydrateformation. Suitable gases include, among others, methane, ethane,propane, butane, and mixtures thereof.

Methods of desalination or purification using buoyant gas hydrates knownprior to Applicant's co-pending application Ser. No. 09/350,906 rely onthe naturally high pressures and naturally low temperatures that arefound in open ocean depths below 450 to 500 meters when using puremethane (or at shallower depths when using mixed gases to enlarge thehydrate stability “envelope”), and the desalination installations, beingfixed to pipelines carrying fresh water to land, are essentiallyimmobile once constructed. In certain marine locations such as theMediterranean Sea, however, the water is not cold enough for therequisite pressure to be found at a shallow enough depth; this wouldnecessitate using a much longer column, which may be impractical.

In addition to the temperature of the seawater, other heatconsiderations are relevant to systems for desalination or purificationof water using gas hydrates. When gas hydrate forms, it gives off heatin an exothermic reaction due to significantly higher heats of fusionthan water-ice. In a hydrate fractionation desalination apparatus, thecold water and high pressures required for natural hydrate formation inthe sea are reproduced within the desalination apparatus. According tothis approach to water desalination or purification, in order for a gasor mixture of gases spontaneously to form gas hydrate when mixed withtreatment water at sufficiently high pressures, the treatment water mustbe of sufficiently low temperature.

Because the stability of hydrate is governed by both the temperature andpressure of the water-to-be-treated in which the hydrate forms, incertain circumstances, only a certain amount of hydrate can be formedbefore the heat generated by the exothermic formation of hydrate raisesthe temperature of the residual water to a level at which hydrate willno longer form. In other words, for a given volume ofwater-to-be-treated, which volume of water initially is suitable for thespontaneous formation of hydrate when hydrate-forming gas is introduced,the formation of hydrate itself can limit the amount of hydrate whichwill form because of the associated rise in temperature. Accordingly,there is a need for methods and systems that overcome this effectiveself-limitation.

Furthermore, laboratory and at-sea experimental experience has shownthat causing gas hydrate to form spontaneously in seawater of suitablepressure and temperature can result in different forms of hydrate wherethe hydrate formation is carried out under different physical chemicalconditions. For example, when the hydrate-forming gas is introduced toopen ocean seawater that is very undersaturated in hydrate-forming-gas,hydrate formation is generally restricted to a zone at the interfacebetween water and gas resulting in the formation of relatively thinaggregates of hydrate. Hydrate formation under these conditions is lessefficient and results in the production of a very large number of smallpieces of hydrate which can have shapes which are not as hydrodynamic aslarger pieces of solid hydrate. The small pieces of hydrate that are nothydrodynamic rise buoyantly at slower rates than larger pieces of solidhydrate, and thus begin (and likely complete) their dissociation atdepths deeper than is desired for optimal conversion of hydrate to waterand gas.

SUMMARY OF THE INVENTION

The present invention represents a significant advance over themethodologies disclosed and taught in the '422 application and itsprecursors. In accordance with the present invention, an apparatus isprovided which allows the hydrate formed in the hydrate formation regionof a desalination fractionation apparatus to be cooled as it rises inthe apparatus. This has the beneficial effect of increasing itsstability at lower pressure and reducing the depth at which the hydratewill begin to dissociate. The present invention provides more efficientmanagement of the thermal energy within the system as a whole.

In an additional embodiment, the present invention also allows a highpercentage of hydrate to be formed in and extracted from a given volumeof water-to-be-treated in a single pass through the desalinationapparatus by cooling the hydrate formation region of the desalinationapparatus. This increases the efficiency of the system in terms of themovement of water-to-be-treated and the amount of fresh water produced.

In accordance with another aspect of the present invention, an apparatusand method for the desalination or purification of water is provided inwhich a hydrate-forming-gas (such as methane or some otherhydrate-forming gas or gas mixture) is introduced to at least part ofthe water to be desalinated or purified under pressure-temperatureconditions in which the hydrate-forming-gas dissolves to nearsaturation. The hydrate-forming-gas-saturated water-to-be-treated isthereafter transported to a physical location where additionalhydrate-forming gas is introduced under pressure-temperature conditionsin which the spontaneous formation of hydrate takes place. Dissolvinghydrate-forming-gas in at least part of the water to be desalinated orpurified prior to hydrate formation improves the efficiency ofconverting hydrate-forming gas to solid hydrate, and improves theconversion of hydrate-forming-gas to larger solid hydrate crystals orhydrodynamic aggregates of crystals of approximately the same size. Bymore efficiently converting the hydrate-forming-gas into larger solidhydrate crystals, this aspect of the present invention allows a moreefficient management of the distribution of thermal energy within theapparatus as a whole and creates a more efficient source of desalinatedor purified water.

In accordance with the present invention, the pre-treatment of the waterto be desalinated or purified with the hydrate-forming-gas may beprovided in desalination or purification apparatus which are land-based,sea-based, or in those that are artificially pressurized, although it isillustrated explicitly herein in just a land-based installation.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described in greater detail in connection withthe drawings (including the drawings presented in the '422 application),in which:

FIG. 1 is a generalized, diagrammatic depiction of a land-baseddesalination installation, as per the '422 application;

FIG. 2 is a diagrammatic, side elevation view of an embodiment of adesalination fractionation column which utilizes positively buoyanthydrate and which may be employed in the installation shown in FIG. 1,as per the '422 application;

FIGS. 3 and 4 are diagrammatic, side elevation views showing twoalternative heat extraction portions of a desalination fractionationcolumn employed in the installation shown in FIG. 1, as per the '422application.;

FIG. 5 is a diagrammatic, side elevation view of another embodiment of adesalination fractionation column which utilizes positively buoyanthydrate and which may be employed in the installation shown in FIG. 1,as per the '422 application;

FIG. 6 is a diagrammatic, side elevation view showing overlapping watervents used in the desalination fractionation column shown in FIG. 5, asper the '422 application;

FIG. 7 is a diagrammatic, side elevation view of yet another embodimentof a buoyant hydrate-based desalination fractionation column employed inthe installation shown in FIG. 1, as per the '422 application, whichembodiment is similar to that shown in FIG. 5.

FIG. 8 is a diagrammatic, side elevation view of an embodiment of adesalination fractionation column which permits the utilization ofnegatively buoyant hydrate and which may be employed in the installationshown in FIG. 1, as per the '422 application;

FIGS. 9 and 10 are schematic, isometric and end views, respectively, ofthe dissociation and heat exchange portion of the desalinationfractionation column shown in FIG. 8, as per the '422 application;

FIG. 11 is a Pressure/Temperature diagram depicting regions of CO2hydrate stability, the CO2 liquidus, and the operating envelope for anegatively buoyant, CO2 hydrate-based desalination system, as per the'422 application;

FIG. 12 is a diagrammatic, side elevation view of a residual fluidreplacement section designed to facilitate washing of the hydrateslurry, as per the '422 application;

FIG. 13 is a diagrammatic, side elevation view of another embodiment ofa desalination fractionation column which permits the utilization of anegatively buoyant hydrate, as per the '422 application, whichembodiment facilitates separation of residual seawater from thenegatively buoyant hydrate;

FIG. 14 is a diagrammatic, side elevation view of a slurry-holding,fluid separation apparatus used in the installation of FIG. 13, as perthe '422 application;

FIG. 15 is a diagrammatic, side elevation view of an embodiment of adesalination fractionation column configured to maintain thehydrate-forming gas at elevated pressure, as per the '422 application;

FIG. 16 is a diagrammatic, side elevation view of an embodiment of amechanically pressurized desalination system configured to usepositively buoyant hydrate, as per the '422 application;

FIG. 17 is a diagrammatic, side elevation view of an embodiment of amechanically pressurized desalination system which is similar to thatshown in FIG. 16 but which is configured to use negatively buoyanthydrate, as per the '422 application;

FIG. 18 is a diagrammatic, side elevation view of an embodiment of amechanically pressurized desalination system configured to use eitherpositively or negatively buoyant hydrate, as per the '422 application;

FIG. 19 is a generalized, diagrammatic depiction of a mechanicallypressurized desalination system located on a ship, as per the '422application;

FIG. 20 is schematic diagram illustrating the components of a systemconfigured to produce liquid carbon dioxide and fresh water usingindustrial exhaust gas as the supply of carbon dioxide to form hydrate,as per the '422 application;

FIG. 21 is a phase diagram illustrating the relativepressure/temperature conditions at which hydrogen sulfide and carbondioxide hydrate form, as per the '422 application;

FIG. 22 is a schematic diagram illustrating a dissociation vessel inwhich carbon dioxide hydrate is allowed to dissociate so as to produceliquid carbon dioxide and fresh water, as per the '422 application;

FIG. 23 is a phase diagram illustrating P-T pathways along which carbondioxide hydrate is allowed to dissociate in the dissociation vesselshown in FIG. 22, as per the '422 application;

FIG. 24 is a graph illustrating the variation in density of liquidcarbon dioxide with varying pressure, as per the '422 application;

FIG. 25 is a diagrammatic, side elevation view of an embodiment of adesalination fractionation column which employs downward flow of inputand residual fluids in the desalination fractionation apparatus topromote cooling of the hydrate and maximize efficiency of the system inaccordance with the present invention;

FIG. 26 is a diagrammatic, side elevation view of another embodiment ofa desalination fractionation column which employs downward flow of inputand residual fluids in the desalination fractionation apparatus topromote cooling of the hydrate and maximize efficiency of the system inaccordance with the present invention;

FIG. 27 is a schematic plan view illustrating the distribution ofdesalination fractionation columns grouped about a residual saline waterreturn shaft in accordance with the present invention;

FIG. 28 is a schematic plan view illustrating a symmetrical distributionof desalination fractionation columns, residual saline water returnshafts, and input water shafts in accordance with the invention; and

FIG. 29 is a diagrammatic, side elevation view of an embodiment of adesalination fractionation column which provides for saturating thewater-to-be-treated with hydrate-forming gas or gas mixtures prior tohydrate-formation in a land-based installation.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The '422 application and Its Precursors

As noted above, the present invention provides a significant advanceover the methodologies and apparatus disclosed in the '422 application.A land-based desalination installation as per the '422 application andits precursors is shown schematically in FIG. 1 in generalized fashion.The installation may be divided roughly into three sections or regions:an intake portion 10; a water purification portion 12; andpost-processing and downstream usage section 14.

The intake portion ˜0 consists essentially of the apparatus and varioussubinstallations necessary to extract seawater from the ocean 16 andtransport it to the desalination/purification installation at region 12,including subaquatic water intake piping 18 and pumping means (notshown) to draw the water from the ocean and pump it to shore forsubsequent processing. Large volume installations can be locatedrelatively close to the sea to reduce the piping distance of the inputwater to a minimum, and establishing the installation as close to sealevel as possible will reduce the cost of pumping against pressure head.

The intake pipeline 18 extends sufficiently out to sea that it drawsdeep water, e.g., from the slope 20 of the continental shelf becausedeep water is more pure and colder than shallow water. Alternatively,water may be drawn from locations closer to land, e.g., from areas onthe continental shelf 22 where the distance across the shallow water istoo great for practice. The precise depth from which water is drawn willultimately be determined by a number of factors, including primarily thespecific embodiment of the desalination fractionation column which isemployed. Ideally, the desalination installation, per se, is located sothat the highest part of the fluid-handling system is at or belowsea-level to reduce the costs of intake pumping.

Additionally, the water may be pretreated at a pretreatment station 24.Pretreatment consists mainly of de-aeration, filtering to removeparticulate matter and degassing, consistent with the requirement thatmaterial necessary for hydrate nucleation and growth not be removed fromthe water.

An embodiment of the purification installation 30, per se, as per the'422 application and its precursors is illustrated in FIGS. 2, 3, and 4,which embodiment utilizes positively buoyant hydrate to extract freshwater from seawater. Seawater is pumped into the installation 130 atwater input 32 and is pumped down to the lower, hydrate formationsection 34 of the installation. The bottom of the hydrate formationsection is no more than about 800 meters deep, and perhaps evenshallower (again depending on the particular gas or gas mixture beingused). A suitable, positively buoyant hydrate-forming gas (or liquid) isinjected into the hydrate formation section at 36, and positivelybuoyant hydrate 38 spontaneously forms and begins to rise through thewater column, as is known in the art.

The hydrate-forming gas can be pumped using sequential, in-line,intermediate pressure pumps, with the gas conduit extending either downthrough the fractionation column, per se, or down through the inputwater line so that gas line pressure is counteracted by ambient waterpressure. As a result, it is not necessary to use expensive, highpressure gas pumps located on the surface. Alternatively, once a gas hasbeen liquefied, it can be pumped to greater depths without furthersignificant compression.

Hydrate formation (crystallization) is an exothermic process.Accordingly, as the positively buoyant hydrate forms and risesautomatically through the water column—forming a hydrate “slurry” ashydrate crystals continue to nucleate and grow as they rise, until thehydrate-forming gas is used up—the surrounding water, which willincreasingly become a concentrated saline “residue,” will be heated bythe heat energy released during crystallization of the hydrate.

Below a certain salinity, the heated residual seawater will have arelatively decreased density and will rise in the column along with thehydrate 38. When the salinity of the residual seawater rises high enoughdue to the extraction of fresh water from it, however, the highly salineresidual seawater will sink to the bottom of the water column. Thishighly saline residual seawater is collected in sump 40 at the bottom ofthe fractionation column and is removed.

As the slurry of hydrate and heated residual seawater rises in thefractionation column as per the '422 application and its precursors,heated residual seawater is removed from the system in heat extractionportion 44 of the fractionation column at one or more points 46, as perthe '422 application and precursors. The heat extraction section 44 isshown in greater detail in FIG. 3. As illustrated in FIG. 3, for onemode of separation of hydrate and slurry as per the '422 application andits precursors, water is pumped from the system as part of the verticalfractionation process. This is accomplished through a two-stage process.An internal sleeve 45 allows a primary separation to take place, as awater trap 49 is formed below the top of the sleeve. Hydrate continuesto rise, while water floods the entire section 44. Water is pumped frombelow the level at which hydrate exits from the top of the sleevethrough fine conical screens 47. These are designed to obstruct thepassage of particulate hydrate. (The screens can be heated periodicallyto clear them of hydrate when flow restriction exceeds design limits.)Residual water is drawn off at a slow enough rate that any hydrate thatmay reside within water drawn toward the screen has a greater tendencyto rise buoyantly than the tendency toward downwards or sidewaysmovement associated with the force of suction of the drawn-off water.Very buoyant gas rises and stays within the column.

An alternative configuration 44′ of the heat extraction zone is shown inFIG. 4. In this configuration, a centrifuge is used to allow a separate,mechanically-driven density fractionation system to operate. In thisconfiguration, a segment 51 of the column is made mobile and capable ofrotary movement. The mobile, rotary centrifuge column segment is carriedby bearings 53 at the base 55 and at intervals along its height to keepit in vertical alignment with the entirety of the column, and to allowit to rotate with respect to the portions 57, 59 of the column above andbelow it. This section is motor-driven, using a hydraulic system 61driven from the surface. Vanes 63 within the centrifuge section willcause the water column to rotate, which vanes are designed based onturbine vane design to cause the hydrate-residual water in the sectionto rotate without turbulence and with increasing velocity toward the topof the section where residual water is extracted. Gravity “settling” orfractionation works here in a horizontal plane, where the heavierresidual water “settles” toward the sides of the column while thelighter, more buoyant hydrate “settles” toward the center of the column.The hydrate continues to rise buoyantly and concentrates in the centerof the centrifuge section. It will be appreciated that more than onesuch centrifuge section may be employed.

As the hydrate rises into the upper, dissociation and heat exchangeregion 50 of the desalination fractionation column as per the '422application and its precursors, the depth-related pressures which forcedor drove formation of the hydrate dissipate; accordingly, the hydrate,which is substantially in the form of a slurry, will be driven todissociate back into the hydrate-forming gas (or mixture of gases) andfresh water. However, regardless of the particular method used toextract the warmed residual seawater, heat energy in the surroundingseawater which ordinarily (i.e., in the prior art with respect to the'422 application and its precursors) would be absorbed by the hydrate asit dissociates is no longer available to the hydrate. Therefore, becauseheat has been removed from the system by extracting warmed residualseawater in the heat extraction portion 44 of the apparatus, a net oroverall cooling bias is created in the upper, dissociation and heatexchange portion 50 of the installation, as per the '422 application andprecursors.

This cooling bias can be capitalized upon advantageously. In particular,as indicated schematically in FIG. 2, water being pumped into the system(at 32) is passed in heat exchanging relationship through the regions ofdissociating hydrate per the '422 application and its precursors. Forexample, it is contemplated that the dissociation and heat exchangeportion 50 may be constructed as one or more large, individualenclosures on the order of one hundred meters across. The input waterwill pass via a series of conduits through the regions of dissociatinghydrate and will be cooled significantly as it does so. In fact,although some initial refrigeration will be required at start-up of theprocess, which initial refrigeration may be provided by heat exchangemeans 52, the installation eventually will attain a steady-statecondition in which the amount of heat energy transferred from the inputwater to the dissociating hydrate is sufficient to cool the input waterto temperatures appropriate for spontaneous formation of hydrate at theparticular depth of the dissociation column.

Ideally, the input water is stabilized at 4° C. or below. This isbecause below that temperature, the density of the water increases,which enhances separation of the hydrate-water slurry formed byinjection of the gas. Additionally, for a given pressure, hydratenucleation proceeds faster at colder water temperatures. During thestart-up period, the system as per the '422 application and precursorsis run in a mode of maximum warm fluid extraction (to create a state ofinduced thermal bias) before equilibrium or steady-state is reached;although the duration of this start-up period will vary depending on theparticular installation parameters, the design goal is that oncesteady-state is reached, the system can be run for extremely longoperating periods without being shut down, i.e., periods on the order ofyears. Controlling residue water extraction, and thus heat removal,maintains a steady-state condition so that the apparatus does not keepcooling to below steady-state operating conditions.

Once the hydrate has dissociated into its constituent fresh water andgas or gases, the fresh water is pumped off, e.g. as at 54, and the gasis captured and recycled. (Provisions may be made for liquefying certaingases where this is desired.) Additionally, a portion of the water inthe dissociation and heat exchange region 50 will be “gray water,” whichis fresh water containing some small portion of salts that have beenremoved from the hydrate by washing of the hydrate with water. Thedistinction between the “gray” or mixed water and pure fresh water isindicated schematically by dashed line 56. The gray water may besuitable for drinking, depending on the salt concentration, or foragricultural or industrial use without further processing. The cold,gray water may be recycled back into the fractionation column, either bypumping it back down to the hydrate formation section 34, as indicatedat 58; or it may be injected back into the concentrated hydrate slurryat a region of the fractionation column-located above the heatextraction portion 44, as indicated at 60, to increase the fluid natureof the hydrate slurry and to aid in controlling overall thermal balanceof the system. Furthermore, providing gray water at 62 to diluteresidual interstitial fluid allows for pre-dissociation washing.

As further shown in FIG. 1, in the post-processing and downstream usagesection 14, the fresh water preferably is treated by secondary treatmentmeans 64. The secondary treatment means may include, for example, finefiltering, gas extraction, aeration, and other processing required tobring the water to drinking water standard.

Moreover, depending on operating parameters such as temperature of thesource water, the amount of residual seawater extracted in the heatextraction section 44, dimensions of the installation, and otherparameters such as viscosities of fluids within the system; depending onbuoyancy of the hydrate relative to all fluids within the system;depending on salinity and temperature of residual water; depending onthe design output requirements of fresh water; depending on salinity andtemperature of input water; depending on the design coolingrequirements; depending on system inefficiencies affecting thermalbalance; etc., the fresh water produced will be significantly cooled.This cooled water can be used to absorb heat from other applications orlocations such as the insides of buildings, and hence can be used toprovide refrigeration or provide for air-conditioning.

Finally, according to the '422 application and precursor methodology,once the seawater has been cycled through the desalination fractionationcolumn and downstream processing applications a desired number of times,the residual, concentrated seawater (which may be highly saline innature) is simply pumped back to sea. Alternatively, it may be retainedfor those who desire it.

With respect to overall design, engineering, and constructionconsiderations for the system, it is contemplated that the desalinationfractionation column 130 will be on the order of 15 to 20 meters indiameter, or even larger. Conventional excavation and shaft-liningmethodologies common to the mining and tunneling industry can be used inthe construction of the column 130. Overall dimensions would bedetermined based on the total desired fresh water production desired andrelevant thermodynamic considerations. For example, one cubic meter ofmethane hydrate has the capacity to warm about 90 to 100 cubic meters ofwater by about 1 C as it forms, and that same cubic meter of hydrate hasthe capacity to cool about 90 to 100 cubic meters of water by about 1°C. as it dissociates. (Mixes of suitable gases have higher heats offusion, which makes the process more efficient.) Required coolingtherefore will, in part, determine hydrate production rates, and hencedimensions of the system and the choice of gas or gases to meet thoseproduction rates.

Preferably, the diameter of the residual fluid removal column segment islarger. This facilitates buoyant, upward movement of the hydrate throughthe water column while first allowing separation of residue water fromthe hydrate in the heat extraction region 44 as per the '422 applicationand its precursors, and then dissociation and heat exchange in thedissociation and heat exchange region 50.

The dissociation and heat exchange region 50 may be constituted not justby a single dissociation “pool,” as shown schematically in FIG. 2, butrather may consist of a number of linked, heat-exchanging devices in anumber of different water treatment ponds or pools. The actual depth,size, throughput, etc. will depend on the production rate, which willdepend, in turn, on the temperature of the input water, the particulargas or gas mixture used to form the hydrate, the rate at which heat canbe removed from the system, etc.

The input of water into the base of the fractionation column can becontrolled by a device (not shown) that alters the input throat diameterso as to facilitate mixing of the gas and water, thereby promoting morerapid and complete hydrate formation. Alternatively or additionally,hydrate formation can be enhanced by creating flow turbulence in theinput water, just below or within the base of the hydrate-forming-gasinjection port 36. It may further be desirable to vary the diameter ofthe desalination fraction column in a manner to slow the buoyant ascentof the hydrate slurry, thereby enhancing hydrate formation.

The dissociation and heat exchange region 50 will be significantly widerand larger than the lower portions of the desalination column. This isbecause, as per the '422 application and its precursors, hydrate will befloating up into it and dissociating into gas and fresh water at a ratethat is faster than that which could be accommodated in a pool that isthe diameter of the column itself. Moreover, the requirement for heatwill be great; if sufficient heat cannot be provided, water ice willform and disrupt the desalination process. Provision for physicalconstriction within a column will hold hydrate below the level where itdissociates freely, thus providing for a control on the amount of gasarriving at the surface. This is done for both normal operational andsafety reasons.

Because the positively buoyant hydrate floats, fresh water tends to beproduced at the top of the section, thereby minimizing mixing of freshand saline water. To inhibit unwanted dissociation, the heat exchangerapparatus per the '422 application and precursors may extend downward tothe top of the residual water removal section. The dissociation and heatexchange pools do not need to be centered over the water column;moreover, more than one desalination fractionation column may feedupward into a given dissociation and heat exchange pool. Similarly,groups of desalination fraction columns can be located close together soas to be supported by common primary and secondary water treatmentfacilities, thereby decreasing installation costs and increasingeconomy.

In addition to large-scale installations, relatively small-scaleinstallations are also possible. For these installations, smallerdiameter desalination columns can be constructed in locations wherelower volumes of fresh water are required. Although overall efficiencyof such systems will be lower than larger scale systems, the primaryadvantage of such small-scale installations is that they can beimplemented using standard drilling methods. Furthermore, mass-produced,prefabricated desalination apparatus sections can be installed in thecasings of drilled holes; this allows the installation to be completedin a relatively short period of time. Capital cost of such aninstallation also is reduced, as fabrication of the components can becarried out on an industrialized basis using mass production techniques.The various operating sections of a smaller-scale installation might bereplaced by extracting them from their casing using conventionaldrilling and pipeline maintenance techniques.

Another embodiment 230 of a desalination fractionation column accordingto the '422 application and its precursors is shown in FIG. 5. In thisembodiment, hydrate formation occurs essentially within a thermalequilibration column 132. The thermal equilibration column 132 has anopen lower end 134 and is suspended in shaft 136. In this embodiment,input water is injected near the base of the desalination column 132,e.g. as at 138; after passing through heat exchange and dissociationregion 150 of the column 230 in similar fashion to the embodiment shownin FIG. 2. Positively buoyant hydrate-forming gas is injected into thelower portions of the thermal equilibration column 132, as at 140, andhydrate will form and rise within the column 132 much as in the previousembodiment. The embodiment 230 is simplified in that heat of formationof the hydrate is transferred to water surrounding the thermalequilibration column 132 within a “water jacket” defined between thewalls of the column 132 and the shaft 136 in which the desalinationfractionation column is constructed. To this end, the hydrate formationconduit preferably is made from fabricated (i.e., “sewn”) artificialfiber material, which is ideal because of its light weight and itspotential for being used in an open weave that greatly facilitatesthermal equilibration between residual saline water within the thermalequilibration column 132 and seawater circulating within the waterjacket.

As is the case with the embodiment shown in FIG. 2, warmed water ispumped out of the system, this warmed water being water which hascirculated within the water jacket. In contrast to the embodiment shownin FIG. 2, however, the intent of removing warmed water from the waterjacket is not to remove so much heat energy that the input water isautomatically cooled to temperatures suitable for formation of thehydrate at the base of the column, but rather it is simply to removeenough heat energy to prevent water within the interior of the hydrateformation conduit from becoming so warm that hydrate cannot form at all.Accordingly, the rate at which warm water is removed from the waterjacket may be relatively small compared to the rate at which warm wateris removed from the heat extraction portion 44 of the embodiment shownin FIG. 2. As a result, it is necessary to supplement the cooling whichtakes place in the heat exchange and dissociation region 150 usingsupplemental “artificial” refrigeration means 152. Operation isotherwise similar to that of the embodiment shown in FIG. 2: fresh wateris extracted from the upper portions of the heat exchange anddissociation portion 150; “gray water” is extracted from lower portionsof the heat exchange and dissociation region 150, i.e., from below theline of separation 156; and concentrated brine is removed from brinesump 141.

To facilitate “settling out” of brine which is sufficiently dense to benegatively buoyant due to concentration and/or cooling, and tofacilitate heat transfer and thermal equilibration, the equilibrationcolumn 132 preferably is constructed, per the '422 application and itsprecursors, with overlapping joints, as shown in FIG. 6. Thisconfiguration permits the buoyant hydrate to rise throughout the column,while cooled, more saline water (brine) that has risen with the hydrateslurry can flow out through the vents 142, as indicated schematically.

The desalination fractionation column installation may be furthersimplified by feeding the input water into the system without passing itthrough the dissociation section 250 of the embodiment 330 shown in FIG.7. If the input water is not sufficiently cold, more artificialrefrigeration will need to be provided by refrigeration means 252, butoperation is otherwise the same as embodiment 230 shown in FIG. 5.

The methods and apparatus according to the '422 application and itsprecursors can be adapted to accommodate negatively buoyant hydrates. Anembodiment 430 of a desalination fractionation “column” configured topermit the use of negatively buoyant hydrate for water purification isshown in FIGS. 8-10. The major difference between this embodiment 430and the preceding embodiments of desalination fractionation columns isthat the heat exchange and dissociation portion 350 of the installationis laterally or horizontally displaced or offset relative to the hydrateformation and heat removal sections 336 and 346, respectively. Thehydrate formation and heat removal sections are similar to those in theembodiments described above.

A number of different operating gases can be employed with thisconfiguration. Low molecular weight gases such as O2, N2, H2S, Ar, Kr,Xe, CH4, and CO2 can all form hydrates under differentpressure-temperature conditions. Each of the different hydrate-forminggas systems will require special design of the hydrate column which istailored to the particular gas used in the installation, but theprinciples of hydrate formation to extract fresh water will remain thesame. Additionally, adding small amounts of additive gas(es) to theprimary hydrate-forming gas may broaden the hydrate stability field inthe same way the methane hydrate stability field is expanded by mixinghigher density hydrocarbon gases with methane.

Although a number of different gases that form negatively buoyanthydrate may be used for hydrate desalination, carbon dioxide and thedesalination column in which it is used are described in the '422application and its precursors to illustrate the design requirements andconsiderations for a desalination system employing hydrate that isnaturally less buoyant than seawater. Carbon dioxide (or gas mixturescontaining predominantly carbon dioxide, referred to simply as “carbondioxide” for simplicity) is an ideal gas to use for a number of reasons.In particular, carbon dioxide does not combust under the physical andthermal conditions encountered in the hydrate desalination apparatus,and is thus virtually hazard-free. Carbon dioxide hydrate is stable atshallower depths than methane hydrate (and about the same as mixed gasmethane hydrate). Even if present dissolved in relatively highconcentrations, carbon dioxide is safe for human consumption and is notoffensive to either taste or smell (as would be the case of H2Shydrate). (In fact, fresh water produced using carbon dioxide can bemade so as to retain some quantity of the carbon dioxide, therebyproviding soda water that is similar to many popular brands but that isdifferent in at least one significant way in that it will contain allthe naturally occurring minerals found in seawater in proportion to theremaining salts not removed during the desalination process.) Carbondioxide hydrate is, like methane, tasteless and odorless. There isconsiderable relatively recent experimental information whichdemonstrate clearly the actual marine behavior of the formation andbehavior of carbon dioxide hydrate. Carbon dioxide is very common andcan be produced locally almost anywhere and is also commonly availableas an industrial waste product—particularly in the exhaust gasesproduced when burning fossil fuel. As further noted in the '422application and its precursors, the higher heat of fusion of carbondioxide hydrate will heat the residual water more quickly than methaneor methane-mixed gases.

Design and engineering of the desalination fractionation column will bedetermined in large measure based on the phase properties of theparticular gas being used. FIG. 11 shows, for example, the carbondioxide hydrate stability regions superimposed over the carbon dioxidephase diagram. The shaded portion of the diagram indicates that carbondioxide hydrate (formed from carbon dioxide gas) is stable at from anupper pressure limit of about 18 atmospheres, just above 0° C., to about40 atmospheres pressure at just above about 8° C. With respect to carbondioxide, per se, the liquidus extends from about 37 atmospheres pressureat just above 0° C., to about 40 atmospheres pressure at just above 8°C. Above the liquidus, carbon dioxide exists as a gas; below theliquidus, carbon dioxide spontaneously compresses to a liquid.

Accordingly, the system is constructed so that, assuming carbon dioxideis used as the operating gas, the carbon dioxide is injected into thehydrate formation portion of the column at ambient temperature andpressure that is within the operating region 450 that consists of theportion of the carbon dioxide hydrate stability zone that lies above thecarbon dioxide liquidus and above the freezing point of water. Thepractical result of this arrangement is that the range of water depthsat which carbon dioxide may be used as the operating gas is relativelysmall and is comparatively shallow. Accordingly, a relatively shallowland apparatus can be constructed, which will reduce constructioncomplexity and cost.

Similar to the embodiments of the '422 application described above,carbon dioxide (or other negatively buoyant hydrate-forming gas, asdesired) is injected near the base of the hydrate formation section 336(e.g., at 352) and mixed with supply or input seawater that has beenchilled by being passed through the heat exchange and dissociationportion 350 and/or by “artificial” refrigeration, as at 354. The carbondioxide hydrate will float only if the formation of the hydrate isincomplete such that a complex, hydrate-gas meshwork is formed. Thiscondition is met when the gas is injected rapidly and in relativelylarge bubbles. The carbon dioxide hydrate isolates carbon dioxide gasbubbles from the surrounding seawater, thereby preventing furtherformation of hydrate. The combined gas/liquid carbon dioxide and hydrateis positively buoyant, even though the hydrate per se is negativelybuoyant (i.e., has a greater specific gravity than the seawater), andfloats upward, as at 356. Additionally, some of the bubbles will burstand new hydrate shells will be formed; hydrate shells with gas bubblespredominantly form new carbon dioxide hydrate rims, which are assistedupward by carbon dioxide gas which tends to adhere to solid hydrateparticles.

The system is designed to produce as much hydrate as possible,consistent with leaving enough warm, lower-density, residual fluid toform a “flux” and to allow extraction of heat by removing the residualseawater in the heat extraction section 346, per the '422 applicationand its precursors. The system furthermore has the capacity for veryrapid liquid or gas injection, which may be in time-sequence burstsrather than being continuous. It is intended that not all gas formhydrate, as noted above, to ensure incomplete formation of hydrate.Thus, larger quantities of gas are required for a negatively buoyanthydrate-based system than for a complete hydrate-forming gas system suchas the positively buoyant hydrate-based systems described above.

As in the case of positively buoyant hydrate-based embodiments,formation of the negatively buoyant (assisted buoyancy) hydrate isexothermic. Accordingly, according to the '422 application andprecursors, heat which is given off during hydrate formation warms thesurrounding, residual seawater, which makes the residual seawater morebuoyant than the chilled seawater which is being input into the lowerpart of the column. The residual seawater therefore moves buoyantlyupward along with the hydrate as new, denser input water is supplied tothe base of the fractionation column, as at 360.

The upward movement of the surrounding residual seawater, as per the'422 application and its precursors, along with the original upwardmovement of the assisted buoyancy hydrate, has a certain momentumassociated with it. This carries the hydrate upward through the columnuntil it reaches a lateral deflection zone 362, where thehydrate/residual seawater slurry is diverted horizontally or laterallyrelative to the hydrate formation and heat removal sections 336 and 346and into the dissociation and heat removal section 350. Thus, eventhough some of the hydrate “bubbles” will burst or crack, therebyreleasing the carbon dioxide gas contained therein and losing buoyancy,the hydrate in large measure continues to move upward and over into theheat exchange and dissociation region of the column 350 due to thismomentum. As the hydrate loses momentum within the heat exchange anddissociation portion 350, it will settle and dissociate into the gas andfresh water, which will separate from residual seawater as described ingreater detail below.

Some of the hydrate, however, will form solid masses without entrappedgas and will sink to the lowermost, sump portion 364 of the column.Concentrated brine will also sink to and settle in the sump portion 364.The sunken hydrate and concentrated residual brine are pumped out of thesump at 365 and separated by appropriately configured separation means366. The waste saline water 368 is disposed of as appropriate, and aslurry consisting of the sunken hydrate is pumped upwardly as indicatedat 370 and is discharged into the heat exchange and dissociation chamber350, e.g. at 372, where the hydrate dissociates into gas and freshwater.

Within the dissociation and heat exchange chamber 350, the hydrate,whether delivered or transported to the chamber via the lateraldeflection portion 362 of the column or pumped from the sump of thedesalination fractionation column 364, will dissociate into fresh waterand the hydrate-forming gas.

To facilitate separation of fresh water from saline water, transfer ofas much hydrate to the upper part of the dissociation and heat exchangechamber 350 as possible is promoted; hydrate is held as high in thedissociation and heat exchange chamber 350 as possible untildissociation of that volume of hydrate is complete; and mixing of thefresh water produced by dissociation and the more saline residual wateris kept to a minimum. The configuration of the dissociation and heatexchange chamber shown in FIGS. 9 and 10 facilitates those objectives.

In particular, the assisted buoyancy hydrate slurry rising through thedesalination fractionation column enters the chamber as at 360 afterbeing diverted laterally at deflection portion 362, as indicatedschematically in FIG. 9. Additionally, hydrate slurry being pumped fromthe sump, per the '422 application and its precursors, is injected intothe dissociation chamber at 372, where it may be placed within specialfluid separation devices. The dissociation and heat exchange chamber isconstructed with a number of canted separator shelves 380 which extendfrom one end of the chamber to the other, as well as from one side ofthe chamber to the other. The canted nature of the shelves allows thedenser saline water to sink and the lighter fresh water to rise withinand between the shelves, thereby minimizing turbidity and mixing ofsaline and fresh water. The separator shelves 380 are canted in thatthey slope downward, both from one end of the chamber to the other aswell as from one side of the chamber to the other. The separator shelveshave pass-through apertures 382 which allow the denser, saline water tosink within the system and the less dense, fresh water to rise withinthe system to the top of the chamber as the hydrate dissociates into thefresh water and gas.

Fresh water, which is cooled due to the cooling bias created by theremoval of warm residual water as described above in connection with thepositively buoyant hydrate embodiments, is removed as at 384 and may beused for cooling as well as for potable water. “Gray” water and salineresidue are removed from lower portions of the heat exchange anddissociation chamber 350, as at 386 and 388, and are handled asdescribed above in the context of the positively buoyant hydrateembodiments, e.g., gray water may be used for drinking or industrialapplications and the saline residue may be recycled back as input intothe base of the desalination fractionation column.

As an alternative to gaseous carbon dioxide, liquid carbon dioxide canbe used to form assisted buoyancy hydrate. At the relatively shallowdepths appropriate to the formation of hydrate for separation of freshwater, liquid carbon dioxide is more buoyant than seawater (although notas buoyant as gaseous carbon dioxide.) By injecting liquid carbondioxide energetically into seawater, a resultant meshwork of hydrate andliquid carbon dioxide is formed which is positively buoyant. Themeshwork mass will rise spontaneously as a whole immediately uponforming and will behave essentially the same as a hydrate meshworkformed from gaseous carbon dioxide and carbon dioxide hydrate.

(Advantages of liquid carbon dioxide over gaseous carbon dioxide stemfrom the fact that once the carbon dioxide is compressed, it can betransported to deeper depths without further compression. Thus,injecting liquid carbon dioxide at depths of five hundred meters ormore—well below the liquidus—is possible without the need for deep,in-line pumps. Moreover, deeper (i.e., higher pressure) injection ofliquid carbon dioxide will promote very rapid crystallization and growthof the hydrate crystals.)

When liquid carbon dioxide is used to form assisted buoyancy hydrate,dissociation is comparatively violent because the unhydrated liquidcarbon dioxide trapped within the meshwork produces large volumes ofcarbon dioxide gas when the mixture rises above the liquidus. Thus, inaddition to the carbon dioxide gas released by dissociation of thehydrate (which occurs above the carbon dioxide liquidus), the extra gasproduced by conversion of the liquid carbon dioxide to gaseous carbondioxide has the potential to cause significant turbulence and mixing.Therefore, flow of the hydrate should be controlled such that it entersthe dissociation section while still within the hydrate stability fieldin order to preclude significant dissociation while residualinterstitial saline water remains in the slurry.

Additionally, where carbon dioxide liquid is used to form assistedbuoyancy hydrate, care should be taken to allow residual fluid to alterits state to gas once the hydrate has risen above the liquidus pressuredepth, but while the hydrate remains stable. This will reduce turbulenceand mixing when the hydrate finally dissociates.

Ideally, according to the '422 application and its precursors, residualsaline water should be replaced by fresh water before the hydrate risesinto the gas-stable zone and then the dissociation area of the carbondioxide hydrate phase diagram (FIG. 11). This can be accomplished usingmultiple water injection points alternatingly arranged between multipleresidual or interstitial water removal sections, as illustrated in FIG.12. In other words, the fluid removal section 44 (FIG. 2) is constructedas an alternating sequence of fresh water injection subsections 412 andfluid removal subsections 414 constructed as shown in either FIG. 3 orFIG. 4. The benefits of removing the interstitial saline fluid includeadditional heat removal; washing of the slurry (i.e., removal ofpollutants or adhering ions or particulate material from the surface ofthe hydrate crystals) by fluid replacement; and direct removal of salineinterstitial water from the hydrate slurry and dilution or replacementof the original saline interstitial fluid produced by the process ofhydrate formation.

Although washing of interstitial water is strongly recommended for theslurry mixture of liquid carbon dioxide and carbon dioxide hydrate—aswell as for any hydrate being used for water purification, wherepossible—so as to minimize turbulence and mixing attributable to theliquid carbon dioxide converting to gaseous carbon dioxide, washing theslurry and flushing saline interstitial fluid therefrom would alsoprovide benefits for any positively buoyant hydrate-based or assistedbuoyancy hydrate-based system. In particular, injecting cold water(either fresh or gray) from the dissociation section into the hydrateslurry will remove additional heat from the hydrate at the same timethat saline interstitial water is flushed from the hydrate slurry.Moreover, multiple residual water flushings will ensure greater freshwater production.

Another embodiment 530 of a desalination fractionation “column”according to the '422 application and its precursors, which isconfigured to utilize negatively buoyant hydrate and which facilitatesseparation of the hydrate and residual seawater, is illustrated in FIGS.13 and 14. The “column” is configured as an asymmetric, U-shapedinstallation, which consists primarily of a seawater, input conduit 432,a hydrate formation and catch sump region 434, and a residue fluid riserconduit 436. As in previous embodiments, the seawater input conduitpasses through a dissociation and heat exchange region 438 which, inthis embodiment, is configured especially as a hydrate “catch basin.” Asin the previous embodiments of the '422 application and its precursors,the input water is passed through the dissociation/heat exchange catchbasin 438 in heat exchanging relationship with dissociating hydrate inorder to chill the input seawater.

The input seawater is pumped to the base 440 of the column, where itturns and flows upward and laterally through elbow portion 442 beforeentering the hydrate formation and catch sump 434. Negatively buoyanthydrate-forming liquid or gas is injected into the input seawater in thehydrate formation and catch sump at 444. (Means 945 for liquefyingcertain gases are provided; residual fluid can be used in a heatexchanger 457 to provide cooling for the liquefication process.)Injection of the gas or liquid is controlled such that hydrate formationis complete (in contrast to incomplete, as in the case of the previouslydescribed, assisted buoyancy embodiment), i.e., such that all gas isutilized to form hydrate. The negatively buoyant hydrate settles to thebottom of the catch sump 434. As the hydrate settles, it displaces theresidual seawater, which is warmed by the heat liberated during hydrateformation. The residual seawater therefore rises buoyantly throughresidue fluid riser conduit 436, and it is pumped out of the system toremove heat and create a cooling bias in the system as in the previouslydescribed embodiments.

In this embodiment according to the '422 application and its precursors,the rate of formation and settling of the hydrate is controlled suchthat it “packs” down to the point of being grain supported. Mechanicalapparatus such as a vibration tray is located on the sloping floor 439of the settling portion of the hydrate-residual fluid chamber 434. Thisconcentrates the hydrate and minimizes residual fluid remaining so thatthe hydrate can be pumped rapidly, as a slurry, from the base of thesump up into the dissociation/heat exchange catch basin 438 via slurrypumping conduit 446. The hydrate slurry is pumped to thedissociation/heat exchange catch basin 438 at a rate that is generallyfaster than the rate at which positively buoyant hydrates rises in thepreviously described embodiments. Decreasing the time required totransfer the hydrate from the formation region (where it is at itsmaximum stability) to the dissociation region (where it is at itsminimum stability) ensures that a greater proportion of the hydrate willdissociate relatively high in the catch basin. This reduces the amountof mixing of fresh and residue water and increases the relativeproportion of fresh water that can be recovered.

Pumped hydrate slurry arrives in the dissociation/heat exchange basin ina concentrated form with little more than intergranular saline waterpresent. Care is taken to allow the saline water to separate downwardand fresh water upward so that there is a minimum of mixing. This isachieved by placing a slurry holder and fluid separator tank in theupper part of the dissociation/heat exchange chamber 438. This allowsthe negative buoyancy hydrate dissociation to take place so that salinewater is delivered to and collects in the lower part of the dissociationchamber 438, in which the slurry holder and fluid separator tank isplaced, without mixing with fresh water.

A preferred slurry holder and fluid separator consists of a fixed,wide-mouthed, upwardly open tank or tanks 450 (FIG. 14) that receive thehydrate slurry from above. Each tank holds the negatively buoyanthydrate from the hydrate slurry transfer system 446 and prevents it fromsinking toward the base of the dissociation chamber 438. The hydrateslurry is delivered by pipes 448 to a number of hydrate spreadersconsisting of vanes or rotating vanes designed to disperse the granularhydrate 468. The negatively buoyant hydrate separates while falling to ascreen shelf 470 in the tank. This allows saline water to sink throughthe screen shelf at the base of the circulating input water intercoolersystem 474, which, according to the '422 application and its precursors,transfers heat from the input water to the dissociating hydrate andfeeds the cooled water downward to be treated.

A number of residual water delivery pipes 475 extend downward from thebase of this slurry holding tank, which allows heavier saline water toflow to the base of the vessel without disturbing the water surroundingthese pipes. Thus, even when the fresh water-saline water interface islocated in the vessel below the slurry holding tank, no mixing occursbetween the residue water purged from each input of hydrate slurrybecause of a physical separation. According to the '422 application andits precursors, the main interface 477 (dashed line) between fresh andsaline water will be located somewhere the lower part of thedissociation/heat exchange chamber 438, where saline water naturallycollects below fresh due to density differences. Saline water is removedat the base of the chamber 480, and provision is also made for graywater removal as at 482.

Multiple slurry holding tanks may be placed within a givendissociation/heat exchange chamber so that the flow of hydrate slurrycan be rapid enough to prevent clogging or freezing up of any one tank.As per the '422 application and its precursors, circulating input watermay be passed first through one slurry holding tank and then throughanother to minimize temperature of the input water as it exits thedissociation/heat exchange chamber.

All fluids will find their relative positions according to naturalbuoyancy or through a process of fractionation. All internal piping inthe vessel can be fabricated from inexpensive plastic or other material.This method of fluid separation may also be installed in thedissociation/heat exchange section of the assisted buoyancy and pumpedsump embodiment shown in FIG. 8.

The slurry pumping conduit 446 may be constructed as a variable volumepipe, in order to permit periodic pumping of hydrate without allowingthe hydrate to settle or move upward slowly. Such a variable volume pipecan be fabricated relatively easily by inserting a flexible sleevewithin the slurry pumping conduit 446 around which fluid can flood whenthe pressure within the liner is reduced.

The injection point 444 of the hydrate-forming liquid or gas ispositioned above the base of the column 440 so that in the event ofincomplete hydrate formation (which would result in the formation ofassisted buoyancy hydrate), any excess gas which does not form hydrate(along with assisted buoyancy hydrate) will rise up the residue fluidriser conduit 436. (Very little hydrate will escape with gas up theresidue fluid riser conduit 436, and any such hydrate will havedissociated prior to arriving at the top of the residue riser section.Therefore, the amount of fresh water “lost” by being transported by suchhydrate will be minimal; recovery of that fresh water is not feasible;and accordingly no connection is provided between the output of theresidue fluid riser conduit 436 and the dissociation/heat exchange catchbasin 438.)

For proper operation of this embodiment, flow rate controls such asconstrictors should be used to keep the rate of flow of fluid throughthe system low enough to keep solid hydrate from being swept up theresidue fluid riser conduit 436. Furthermore, the design of the hydrateformation and catch sump 434, as well as the lower portion of theresidue fluid riser conduit, should be designed to facilitate “clean”separation of the hydrate from the residue fluid. Accordingly, thehydrate formation and catch sump 434 is designed to impart lateralmovement to the residue fluid as well as to permit upward movementthereof. This causes the hydrate/residue fluid mixture to move initiallywith a relatively small upward component, which facilitates settling outof the hydrate and which is in contrast to the previously describedembodiments, which provide more vertically oriented fluid movement thatis comparatively turbid and which have poorer settling and separationcharacteristics.

In the embodiments of the '422 application described thus far, theweight of the column of water creates the pressures required for hydrateformation. In those embodiments, the minimum-pressure depth at whichhydrate is stable is far greater than at sea level, where the pressureis one atmosphere. Accordingly, the hydrate begins to dissociate atrelatively elevated pressures.

Various ones of the embodiments described above may be modified so as tocollect the fresh water released from the hydrate and to capture thereleased gas at the region of the fractionation column where thedissociation takes place, rather than at the top of the column (surfacelevel; one atmosphere ambient pressure), with certain resultantadvantages. In particular, relatively large volumes of hydrate-forminggases and gas mixtures are required to desalinate large volumes ofwater. Therefore, if the gas is captured, processed for re-injection,and stored while maintained at elevated pressures (e.g., the pressure atwhich the hydrate begins to dissociate), the volume of gas that must behandled will be much smaller than would be the case if the gas wereallowed to expand fully as it rises to the surface and pressure drops toatmospheric. Additionally, if the hydrate-forming gas is keptpressurized, raising its pressure to the pressure required for injectionin the hydrate-forming section requires far less recompression of thegas and hence is less costly.

An embodiment 600 as per the '422 application and its precursors inwhich dissociation and gas capture and processing are controlled so asto be kept at elevated pressure is illustrated in FIG. 15. In thisembodiment, a physical barrier 610 extends across the fractionationcolumn and blocks the upward movement of the hydrate slurry. Thelocation of the barrier 610 depends on the stability limits of theparticular hydrate-forming substance used, but will be above the regionof hydrate stability (i.e., at lesser pressure depth). As the hydratedissociates, the released gas forms a pocket at trap 620 and enters agas recovery and processing system 626 while still at a pressure depthconsiderably greater than one atmosphere surface pressure. (The gasprocessing system 626 may contain means for liquefying certain gases.)The gas is processed and re-injected into the hydrate formation section628 at 629 in the same manner as in the previously describedembodiments, except the gas system is maintained at considerably higherpressure.

The hydrate dissociation section 630 extends downward to some particulardepth determined by the particular hydrate-forming gas being used.Because the hydrate dissociates under “controlled,” elevated pressure,the dissociation reaction will proceed generally more slowly than in theabove-described embodiments. Therefore, the heat exchanger 632 presentin the dissociation/heat exchanger section (as described in connectionwith previous embodiments) is designed to accommodate the particular,slower reaction rates. As per the '422 application and its precursors,input water 634 is passed through the dissociation/heat exchange sectionin heat exchanger 632 and is injected into the base of the desalinationfractionation column at 636, as in previously described embodiments.

One or more fresh water bypass pipes 640 communicate with thedissociation region at a point 641 located above the fresh water/salinewater interface 644 but below the upper boundary 648 of the hydratestability field. The pipe(s) 640, which are screened or otherwiseconfigured to prevent hydrate from entering them, deliver fresh waterreleased from the hydrate to fresh water accumulation region 666. A graywater return pipe 650 allows denser, more saline gray water to flow backdown into the saline fluid below the fresh water/saline water interface644. More highly saline residual water and/or negatively buoyant hydrateis drawn from the sump 654 and processed or removed as at 658, as inpreviously described embodiments. Output fresh water, some of which maybe returned to the fluid removal section for purposes of washinginterstitial saline water as described above (not shown), is drawn offat 660, near the top of the fresh water accumulation region 666 and wellabove the physical barrier 610.

It is contemplated that the physical barrier 610, the fresh water andgray water return pipes 640, 650, and the heat exchanger in thedissociation/heat exchange section 630 may be configured such that theirpositions can be varied, thereby permitting different hydrate-formingliquids, gases, or gas mixtures to be used in the same installation. Thephysical barrier 610 and heat exchanger might be vertically adjustable,whereas a series of bypass and return pipes 640, 650 having differentinlet locations can be provided and opened and closed remotely usingsuitable inlet and outlet valves. In this manner, changing from onehydrate-forming substance to another can be effected very quickly andconveniently.

By holding and fully processing for re-injection the hydrate-forming gaswhile it is still under pressure, considerable economies of operationcan be achieved. The variation in the pressure of the liquid or gas,from that required for formation of the hydrate down to that at whichfresh water is released from the hydrate, can be kept to a minimum.This, in turn, minimizes the energy cost associated with pumping thecaptured hydrate-forming gas from above the dissociation/heat exchangesection back down to the hydrate-forming section at the base of theapparatus, particularly considering the fact that, percentagewise, thegreatest change of pressure in a hydraulic column (such as any of theabove-described embodiments) takes place in the upper portions of thecolumn. Moreover, the volume of the gas to be handled (and accordinglythe size of the gas handling equipment and facility) will also bereduced significantly.

As an alternative (not shown) to the configuration shown in FIG. 15, theupper part of the desalination fractionation column can be sealed andpressurized by means of an associated hydraulic standpipe, therebycausing pressures within the apparatus near the surface to be equivalentto the pressure-height of the standpipe. Where the standpipe isimplemented in tall structures (such as adjacent buildings near thedesalination facility), relatively high pressures can be created in thetopmost part of the dissociation/heat exchange section, which is atground level.

In further embodiments of the invention as per the '422 application andits precursors, the water may be desalinated or-purified inself-contained, mechanically pressurized vessels. Such embodiments offera number of advantages, including the fact that the installations can beof various sizes and shapes to suit local conditions, containmentconstraints, and fresh water requirements. Moreover, whereas thepreviously described embodiments are relatively large-scale andtherefore are of a fixed, permanent nature, self-contained, pressurizedembodiments can be more temporary in nature in terms of theirconstruction and their location. Individual pressurized installationscan occupy relatively small spaces and produce fresh water efficiently,even in low volumes. Such installations can be fabricated at centralmanufacturing facilities and installed on site with a minimum of localsite construction, which site might be a building or even a ship orother mobile platform.

A mechanically pressurized installation per the '422 application,configured to use positively buoyant hydrate to extract fresh water fromseawater, is illustrated in FIG. 16. Input water is pumped andpressurized from input pressure to the operating system pressure by pump704. The water enters the pressurized hydrate formation and separationvessel 710 at water input 711, and a suitable, positively buoyanthydrate-forming substance is injected at injection point 712. (Means 713for liquefying certain gases are provided where this is advantageous tothe desalination process.) Positively buoyant hydrate 714 spontaneouslyforms and rises through the residual water, as in previously describedembodiments, to the top of the vessel 710 where it accumulates andconcentrates.

The buoyant hydrate slurry is subsequently admitted into transfer andwashing section 720, and then into the dissociation/heat exchange vessel722; (Flow of the hydrate slurry is regulated by valves 734.) While inthe transfer and washing section 720, the hydrate may be washed of theresidual, intergranular saline fluid using fresh water 726 tapped fromthe fresh water output 728. More than one wash cycle may be used tocompletely flush residual fluid, although the number of washings willdepend on the effectiveness of separation through fractionation (whichmay vary for different gases and gas mixtures) and the nature of thecrystalline fraction of the slurry. In some cases, no washing may benecessary.

Pressure is maintained in the hydrate formation and separation vessel710 and in the dissociation/heat exchange vessel 722 by pressure balancereservoir systems 732 (one for each vessel), and movement of fluid fromone vessel to the other is controlled by varying pressure and using thein-line valves 734. The systems 732 each have a pressure pump 733 and adiaphragm or gas-fluid interface 736, which are used to raise and lowerpressure in each vessel. Pressure in the vessels is controlled so thatthe hydrate remains stable as hydrate until it is finally collected andconcentrated at the top of the dissociation vessel 722. This is becausepremature dissociation will release considerable amounts of gas andtherefore will cause undesired mixing. Moreover, pressure conditions inthe dissociation vessel should be controlled to minimize turbulence inthe fluid-gas mixture and to promote efficient separation of saline andfresh water.

The dissociation and heat exchange vessel 722 may be constituted by anumber of linked, heat-exchanging devices in a number of different watertreatment chambers. The actual size, throughput, etc. will depend on theoverall system production rate which, in turn, will depend on thetemperature of the input water, the particular liquid, gas, or gasmixture used to form the hydrate, the rate at which heat can be removedfrom the system, etc. Fractionation, concentration, separation, drying,and re-use of the hydrate-forming gas takes place in the same manner asin the previously described embodiments. Additionally, heat produced byliquefying hydrate-forming gas can be absorbed and removed using heatexchangers containing residue or saline fluids.

It will be appreciated that the mechanically pressurized process per the'422 application and its precursors is inherently less continuous thanthe previously described embodiments and is essentially a batch process.Pressure in the system is controlled so as to simulate the pressurevariation in the previously described embodiments: thewater-to-be-treated is pressurized and injected into the apparatus, andthen pressure is raised and lowered to control the rate of the hydrateformation and dissociation reactions.

Mechanically pressurized embodiments provide increased versatility inthat pressures may be controlled to provide the optimum pressures forformation of hydrate and to control the rate of dissociation. Moreover,different liquids, gases, and gas mixtures can be used within the sameapparatus, and the same water can be processed more than once usingdifferent liquids, gases, and gas mixtures.

A further mechanically pressurized embodiment 800 per the '422application and its precursors, which embodiment utilizes negativelybuoyant hydrate to extract fresh water from water-to-be-treated, isshown in FIG. 17. Input water is pumped from input pressure up to theoperating system pressure and into the pressurized hydrate formation andseparation vessel 810 by pumps 804, and a suitable, negatively buoyanthydrate-forming gas is injected at injection point 812. (Means 813 forliquefying certain gases may be provided.) Negatively buoyant hydrate814 spontaneously forms and sinks through the residual water, asdescribed in connection with previously described negatively buoyanthydrate embodiments per the '422 application and its precursors, andcollects and concentrates in gated sump isolation sections 816, whichare opened and closed to control passage of the hydrate therethrough.

As in the previously described mechanically pressurized embodiment,pressure is maintained in the system by pressure balance reservoirsystems 832 (one for each vessel), and movement of the fluid can becontrolled by varying the pressure in the system compartments. Pressurepumps 830 and diaphragms or gas-fluid interfaces 836 are used to raiseand lower pressure in each vessel independently.

As the hydrate slurry passes through the transfer and washing section820 and into the dissociation/heat exchange vessel 822, it may be washedof the residual, intergranular saline fluid with fresh water tapped fromthe fresh water output 828, which removes salt from the hydrate slurryprior to dissociation.

Subsequently, the hydrate is permitted to flow downward from thetransfer and washing section 820, and into the hydrate dissociation andheat exchange vessel 822, where it dissociates and fresh, gray, andsaline water are removed. Heat exchange between the input water and thedissociating hydrate slurry occurs as described in previous embodimentsas per the '422 application and its precursors. Dissociation takes placeunder controlled pressure conditions to minimize turbulence in thefluid-gas mixture and to promote efficient separation of saline andfresh water.

A slurry holder and fluid separator tank 860 is provided in the upperpart of the dissociation/heat exchange vessel 822 and is similar inconstruction to that described above and shown in FIGS. 13 and 14. Thetank 860 minimizes mixing of fresh and saline water by providing aconduit for the residual saline water to sink to the bottom of thevessel, which conduit isolates the saline water from the lower densityfresh water.

As in the case of the mechanically pressurized, positively buoyanthydrate embodiment of the '422 application and its precursors, thedissociation and heat exchange vessel 822 may be constituted by a numberof linked, heat-exchanging devices in a number of different watertreatment chambers. The actual size, throughput, etc. will depend on theproduction rate which, in turn, will depend on the temperature of theinput water, the particular liquid, gas, or gas mixture used to form thehydrate, the rate at which heat can be removed from the system, etc.Fractionation, concentration, separation, drying, and re-use of thehydrate-forming substance takes place in the same manner as in thepreviously described embodiments of the '422 application and itsprecursors.

Another embodiment 900 according to the '422 application and itsprecursors, which embodiment provides greater versatility by usingeither positively or negatively buoyant hydrate to extract fresh waterfrom seawater or polluted water, is shown in FIG. 18. Pumps P andin-line valves 914 are provided throughout the system. Operation,depending on the particular hydrate-forming substance used, is asdescribed in the pressurized vessel installations using eitherpositively or negatively buoyant gas hydrate.

This embodiment is useful where the gas or gas mixture supply isuncertain as a variety of gases may be used. Embodiments of this typecould be useful in disaster relief or in expeditionary militaryactivity, or at any place where a temporary supply of fresh water isrequired without a significant construction requirement. This embodimentcontains all the attributes of both the positive and negative buoyancyhydrate, mechanically pressurized desalination fractionationembodiments, including use of fresh water 926 from the fresh wateroutput 928 to flush residual saline water. Multiple liquid or gasinjection points 908 are provided, as well as provision for handlingeither positively or negatively buoyant hydrate. In particular, multiplepumping units P and fluid control valves 914 are provided to direct theflow of fluids and hydrate slurries in fluid control and washing units916 and hydrate slurry control units 918. The gas processing system 944includes means for liquefying certain recovered gases and gas mixtures.

As in the above-described embodiments in which the weight of the columnof water generates the requisite pressures, any of the mechanicallypressurized vessel installations may be simplified by feeding the inputwater into the system without passing it through the dissociationsection for heat exchange. More artificial refrigeration will need to beprovided, but operation will otherwise be the same as for the positiveand negative buoyancy hydrate embodiments shown in FIGS. 16 and 17 andthe “combined” pressurized apparatus as shown in FIG. 18.

As noted above, mechanically pressurized embodiments of the inventionmay be extremely mobile. In the case of a ship-borne installation (FIG.19), for example, water-to-be-treated is processed as described inprevious embodiments, but in a smaller and more compact installationbuilt right on board the ship 1000.

Negatively buoyant hydrate formed from a liquid or gas (such as carbondioxide) that is non-combustible at the pressures and temperatures ofthis system and the surrounding ambient conditions is preferred,especially where installations are placed in ships or where the handlingof combustible gases constitutes a hazard.

In the ship-borne embodiment as per the '422 application and itsprecursors, some of the heat produced by the hydrate formation reactionis extracted by heat exchangers in the hydrate formation andconcentration vessel (which is possible because of the immediate accessto seawater), and further heat is extracted from the hydrate slurry inthe hydrate slurry transfer system. This pre-dissociation heatextraction maximizes the cooling effect of the hydrate dissociationbecause removing heat in addition to that removed with the residualtreated fluid allows dissociation to begin with the hydrate slurry at alower temperature than would exist otherwise. Thus, the fresh waterproduced will be significantly cooled. This cooled water can be used toabsorb heat and hence can be used to provide refrigeration orair-conditioning. The fresh water is treated as described in previousembodiments, and warmed residual water may be used as a low-grade heatsource (although it is more likely to be pumped back to the sea).

Installation aboard a ship is ideal for the mechanically pressurizedhydrate fractionation method of desalination. This is because theresidual treated water can be returned to the sea immediately, therebymaximizing efficiency of the heat-removal process. The water intake forthe desalination ideally would be placed as low on the keel 1010 of theship as possible to separate intake and residual water return and tominimize uptake of pollutants, which in the case of oil-based productsand many industrial chemicals either float or are usually found inincreasing proportion closer to the sea surface.

Aboard ship, the return fluid can have multiple outlets 1020, whichallows it to be returned to the sea closer to the surface where thewarmer water will float well away from the water intake. In addition,movement of the ship creates considerable turbulence which will promotemixing of the residual water and near-surface water when the ship isunder way. When the ship is tied up, water from a shore source can beused or the system can be recycled with fresh water to minimize residualwater return, and the desalination fractionation system can be operatedat a minimal level, i.e., at a level just sufficient for the thermalbalance required for normal operation to be attained quickly. Where theship is moored or otherwise maintaining a static position, the residualwater can be returned to the sea directly. Wind and tide can be takeninto consideration to select the return outlet utilized so as tominimize environmental impact and allow the residual water to be carriedaway from the ship most efficiently.

Similar compact installations can be fabricated as pre-packagedcomponents that can be airlifted or easily flown and trucked to aparticular site—for instance, immediately following a disaster such asan earthquake—and assembled rapidly. Where temporary or mobileinstallations are operated, more compact versions of the intake,outfall, and gas processing apparatus similar to that described for FIG.1 are employed. These can be specially designed for light weight, easeof deployability, and ability to operate in a variety of conditions.Power generating units or power cables suitable for drawing electricityfrom any inshore powerboat or other supply are also part of the mobileapparatus, and possibly also part of larger temporary facilities.

Pressurized vessel desalination fractionation installations can also bemounted on pallets for shipment in aircraft or ships or in standardcommercial shipping containers (for which cargo handling equipmentexists world-wide) to facilitate air and road travel. They can bemounted on vehicles or set up on a pier, or anywhere near seawater orother water to be desalinated or purified.

Finally, once the seawater has been cycled through the pressurizedvessel desalination fractionation column and downstream processingapplications a desired number of times as per the '422 application andits precursors, the residual seawater is simply pumped back to sea orretained for those who desire it.

As noted above, one reason carbon dioxide is an ideal gas to use forhydrate desalination or purification is that it is extremely common and,in fact, is typically found in industrial waste products—particularly inthe exhaust gases produced when burning fossil fuels. For the most part,the exhaust produced when burning fossil fuels typically contains watervapor, hydrogen sulfide, carbon dioxide, carbon monoxide, and nitrousoxides (NOx) and nitrogen which passed through the combustion process inaddition to soot and other particulate waste matter.

When using industrial exhaust as the source of carbon dioxide fordesalinating or purifying water—such purification may be the primaryobjective of the process, e.g., to produce potable water, or it may besimply a means for capturing carbon dioxide from the waste gas emissionsfor purposes of reducing “green house gases” and their associatedenvironmental harm, with the production of fresh water being a “side”benefit—certain provisions for pre-processing the gas must be made. Inparticular, an installation 1100 according to the '422 application forsimultaneously capturing carbon dioxide from industrial waste gases andproducing desalinated or purified fresh water is illustrated in FIG. 20.Exhaust gas 1102 produced by the combustion of fossil fuels inindustrial plant 1104 is fed through dry processing/preprocessing means11105. The raw exhaust gas is pretreated using filters, absorbents,electrostatic means, chemical sorption techniques, and/or catalysis toremove most of the non-carbon dioxide components. The exhaust gas mustalso be cooled substantially in order for the carbon dioxide hydrate toform, and the preprocessing means 1105 may include means 1106 for suchcooling, e.g., heat exchangers through which the exhaust gas flows.

Before being introduced into hydrate formation vessel 1111, the exhaustgas passes through washing/hydrate-forming prechamber 1108. Prechamber1108 may be a largely water-filled chamber that can be pressurized. Whenthe prechamber 1108 is, in fact, so pressurized, the exhaust gas passesthrough the water in the chamber under pressure conditions sufficient toform hydrogen sulfide hydrate. As illustrated in FIG. 21, hydrogensulfide hydrate will form at pressure/temperature conditions well abovethose required for carbon dioxide hydrate to form, and therefore themajority of the hydrogen sulfide components of the exhaust gas notremoved in the previous, preliminary processing can be removed byforming hydrogen sulfide hydrates in the prechamber 1108 and thenremoving it, e.g., in a solid hydrate flush.

The washing residue from the exhaust gas will consist of solids formedfrom soot, partially combusted hydrocarbons, small amounts of metals andsalts, and, when the prechamber 1108 is suitably pressurized and thereis hydrogen sulfide in the exhaust gas, hydrogen sulfide hydrate. (Thesolids will constitute only very small proportions of the exhaust gas.)When it is produced, the hydrogen sulfide hydrate subsequently willdissociate into hydrogen sulfide and water; with the attendantproduction of sulfuric acid and other hydrogen-oxygen-sulfur compounds,and along with the remaining solid waste will constitute hazardousmaterial in concentrated form which needs to be disposed of usingappropriate means that will be known to those having skill in the art.

The density of hydrogen sulfide hydrate, which has not been studiedextensively, has been calculated to be 1.05 g/cc and has been measuredat between 1.004 g/cc at 1 Mpa and 1.087 g/cc at 100 Mpa. Thus, itsdensity is very close to that of fresh water. Therefore, if theprechamber 1108 is filled with water, the hydrogen sulfide hydrate willnot separate from the water in the prechamber 1108 by sinking,particularly when large volumes of gas are rising quickly through theprechamber. Under conditions of low turbidity with gas bubbles risingthrough the water bath, the hydrogen sulfide hydrate will tend to risenear the surface of the water in the prechamber 1108, where it willaccumulate and form a hydrate-rich layer through which the exhaust gasbubbles. On the other hand, because the large volume of exhaust gasrising through the water in the prechamber 1108 will cause extremeturbidity, the hydrogen sulfide hydrate is unlikely to separate outnaturally by means of gravity fractionation. Therefore, in the case of awater-filled prechamber 1108, the prechamber will have to be flushedperiodically, which precludes operating the process on a continuousbasis.

Alternatively, if the prechamber 1108 is pressurized and largelygas-filled, a spray of water 1109 taken from the heat exchange system1107 can be used to wash and cool the exhaust gas efficiently whileallowing hydrogen sulfide hydrates to form. The water spray fills allbut the lower part of the prechamber with a mist of droplets that fallto the bottom of the vessel. (Water evaporating out of the mist andsubsequently passing into the hydrate formation vessel 1111 would simplybecome part of the fresh water product.) Solid matter and any hydrogensulfide hydrate that forms is separated from the wash water, which isheated as it cools the exhaust gas, by means of separation filter 1110.(A separation filter 1110 may also be used in an embodiment per the '422application in which the prechamber 1108 is pressurized andwater-filled, as described immediately above, to filter the wash water.)The solid waste will consist of concentrated hazardous materials thatmust be disposed of according to recognized practices, and the heatedwash water is passed back into the heat exchange system 1107.

After it has been cooled sufficiently by intercoolers 1113 for hydrateto form spontaneously under operational pressure and temperatureconditions, e.g. by processes as described previously for cooling theinput water, the fully preprocessed exhaust gas 1102′ then passes intohydrate formation vessel 1111. The hydrate formation vessel 1111 ispressurized and is configured essentially the same as the pressurizedhydrate formation vessel shown in FIG. 17 or, to the extent it isconfigured for the production of negatively buoyant hydrate, thepressurized vessel shown in FIG. 18. As is the case with those twoembodiments, saline or polluted water and the exhaust gas containing thecarbon dioxide are introduced into the hydrate formation vessel 1111 andpressure and temperature conditions are controlled so as to form carbondioxide hydrate, as described previously. (As in the previousembodiments, this will be done on a batch basis as opposed tocontinuously.) Residual gases such as nitrogen oxide are purged from thesystem.

The carbon dioxide hydrate 1112 is then transferred to pressurizedhydrate dissociation vessel 1114, which may be located below the hydrateformation vessel 1111 as shown in FIG. 17 or, as shown in FIG. 20,adjacent to the hydrate formation vessel (similarly to as shown in FIG.18). The hydrate dissociation vessel 1114 is shown in greater detail inFIG. 22. It is generally similar to the pressurized hydrate dissociationvessels shown in FIGS. 17 and 18, but differs in that a heating element1115, which preferably constitutes part of the system's heat exchangesubsystem 1107, is provided along with an additional outlet 1120 for theremoval of liquid carbon dioxide from the vessel. Concentrated carbondioxide hydrate slurry is introduced into the vessel 1114, as describedpreviously, and the introduction of it into the vessel is controlled bymeans of valve mechanism 1122 so that the carbon dioxide hydrate slurryis introduced on a batch basis. (This is necessary because the pressurein the hydrate dissociation vessel will rise to greater than that in thehydrate formation vessel 1111.) Alternatively, according to the '422application, in-line slurry pumps can be used to maintain higherpressure in the downstream hydrate dissociation vessel. The hydrate isheld in the upper part of the vessel 1114 by a screen 1113 within thetray 1124, where the hydrate dissociates.

By controlling the temperature and pressure conditions within the vessel1114, dissociation of the carbon dioxide hydrate is managed so as toproduce fresh water (as described above in connection with the previousembodiments of the '422 application and its precursors) and liquidcarbon dioxide (in contrast to gaseous carbon dioxide). In particular,as shown in FIG. 23, when the hydrate is introduced into thedissociation vessel 1114, it is introduced under pressure andtemperature conditions in the hydrate stability field in the vicinity ofpoint A, i.e., under conditions at which the hydrate remains stable.(Pressure within the dissociation vessel 1114 may be controlled by meansof a pneumatic standpipe 1126 by admitting an inert gas into thestandpipe using valve 1128.) The temperature within the dissociationvessel 1114 may then be permitted to rise, e.g., by absorbing heat fromthe surroundings or, more preferably, is caused to rise by activelyadding heat removed from the hot exhaust gas back into the system viaheat exchanger 1115. Alternatively, the temperature may be caused torise advantageously by passing the hot exhaust gas around thedissociation vessel 1114 through conduits (not shown) before the exhaustgas is preprocessed in means 1106 and 1108.

As heat being input from the heat exchange system causes the temperatureto rise, the system moves to the right along P-T path 1130 and crossesphase boundary 1132 as per the '442 application, at which point thehydrate dissociates into carbon dioxide gas and fresh water. Because thevessel is sealed, however, as the hydrate continues to dissociate, thepressure and accordingly the temperature continue to rise. As thetemperature and pressure increase, the system continues to move towardthe right along P-T path 1130, but the pressure rises at a sufficientrate that the P-T curve 1130 along which the system moves crosses belowthe carbon dioxide liquidus, and the carbon dioxide gas condenses tocarbon dioxide liquid, as illustrated at point B. (Point B representsjust an example of the system temperature/pressure conditions at whichdissociation is complete; the exact conditions are less important thanmaking sure that the final pressure/temperature conditions within thehydrate dissociation vessel lie within the field of stability for liquidcarbon dioxide.) In operation, the system may pass relatively quicklythrough the portion of the phase diagram in which carbon dioxide gasexists. In those instances, the hydrate will essentially dissociatedirectly into carbon dioxide liquid and pure water.

Alternatively, per the '442 application, by controlling thepressurization of the dissociation vessel 1114 using the gas valve 1128,pressure can be increased sufficiently fast so that the carbon dioxidenever enters the gas phase. The system will then move along P-T curve1130′ through the lower, hydrate stability zone 1132 and directly intothe liquid carbon dioxide/water zone 1134. In either case, however, theresult is the essentially immediate production of liquid carbon dioxideand fresh water.

Non-hydrate-forming-components of the exhaust gas, such as nitrogenoxides and nitrogen, will be released from the system throughvalve-controlled purge system 1150 once the liquid carbon dioxide andwater are removed from the system.

(As yet another, less preferred alternative disclosed in the '442application, if pressure in the system is controlled so as not to riseas high as in these two embodiments, gaseous carbon dioxide instead ofliquid carbon dioxide will be released. In that case, the gaseous carbondioxide can be removed from the hydrate dissociation vessel 1114 at therelatively high gas pressures of the vessel but below those required forhydrate stability (e.g., two hundred to three hundred atmospherespressure) and compressed to liquid carbon dioxide at relatively lowtemperature using industry standard apparatus and methods. Compressingthe pressurized carbon dioxide from the already pressurized gaseousstate to the liquid state would still be easier—and hence lessexpensive—than the case where gaseous carbon dioxide is compressed toliquid carbon dioxide from approximately normal atmospheric pressure.)

As illustrated in FIG. 24, at the pressure and temperature conditionsthat exist at the dissociation end point B, the liquid carbon dioxidewill be positively buoyant with respect to the fresh water produced fromdissociation of the carbon dioxide hydrate. Moreover, the liquid carbondioxide and the fresh water are essentially immiscible. Accordingly, theliquid carbon dioxide will float on top of the fresh water released bythe hydrate, with an interface 1152 (FIG. 22) between the two. The freshwater is then extracted by valve-controlled outlet 1154, and the liquidcarbon dioxide is extracted by valve controlled outlet 1120. The freshwater will be saturated with dissolved carbon dioxide and may containmicro-droplets of liquid carbon dioxide that can be withdrawn underpressure. The fresh water may then be used for any desired purpose,e.g., drinking water, and the carbon dioxide may be transported awayquite conveniently in its liquid form and either used in a variety ofexisting commercial applications or disposed of (e.g., by being pumpedto great ocean depths for sequestration.) Any residual saline water,which will settle to the very bottom of the system, is extracted via avalve-controlled outlet 1156, as described previously, and disposed of.Preferably, even though the carbon dioxide and water are essentiallyimmiscible, a conduit 1158, arranged to carry the more dense fresh waterdown below the less dense liquid carbon dioxide, is provided extendingfrom the bottom of the tray 1124 so that the separated water can flowdownward without having to flow through the carbon dioxide.

Additionally, according to the '422 application, FIG. 22 shows thesystem input water as passing through the dissociation vessel 1114 as inthe previously described embodiments. It will be appreciated, however,that because heat is being added to the system through the heat exchangesystem 1007 so as to move the temperature to the right in the phasediagram shown in FIG. 23, less heat will be absorbed out of the inputwater by the endothermic dissociation of the hydrate, and therefore theinput water will not be chilled to the same extent as in the previouslydescribed embodiments. Accordingly, additional supplemental cooling maybe necessary in order for the hydrate to be formed.

Finally, with respect to the '422 application, although just a singleone of each is shown, more than one dry processing/preprocessingapparatus 1105, prechamber 1108, hydrate formation vessel 1111, and/orhydrate dissociation vessel 1114 may be employed in a singleinstallation. For example, it may be desirable or even necessary tocycle the exhaust gas a number of times prior to introducing it into thehydrate formation vessel 1111 in order to reduce the levels ofnon-carbon dioxide materials which otherwise might cause the water totaste poor.

The Present Invention

As noted above, the present invention represents a significantimprovement over the desalination systems disclosed in the '422application and its precursors. One embodiment of a desalinationfractionation apparatus 1500 in accordance with the present invention isillustrated in FIG. 25. The apparatus 1500 includes a hydrate formationsection 1550 located in the lower portion of fractionation shaft orcolumn 1510, and a hydrate dissociation and separation section 1570(including dissociation-region 1512), where hydrate actually dissociatesinto its constituent hydrate-forming gas and fresh water. A hydrateconcentration section 1560 is located above the hydrate formationsection, where hydrate that has essentially floated up “out of” theresidual fluid concentrates and the slight amount of remaining residualwater of elevated salinity is displaced by fresher water descending fromthe dissociation and separation section 1570.

Hydrate-forming gas 1502 is introduced into the fractionation column1510 via input gas line 1503, which can be a pipe or any other suitablestructure for carrying gas as known to persons skilled in the art. Theinput water-to-be-treated 1450, is introduced into the fractionationcolumn 1510 via water intake pipe 1530. In this particular embodiment,input water 1450 is introduced into the fractionation column above thelevel at which the hydrate-forming gas 1502 is injected.

As explained in the '422 application and above, the input water istypically cooled to a temperature at which, for a given pressure depth,conditions exist for the spontaneous formation of hydrate in thepresence of appropriate hydrate-forming gas. In addition, thewater-to-be-treated may be treated prior to hydrate-formation so thatthe amount of gas in solution that aids the formation of the requiredtype of hydrate is increased or brought to some desired concentration,as explained further below. After the suitable, positively buoyanthydrate-forming gas is injected into the hydrate formation section 1550,positively buoyant hydrate 1464 spontaneously forms and begins to risethrough the water column.

As explained above, the hydrate formation is an exothermic reaction, andthe heat liberated by that reaction tends to heat the surroundingresidual fluid in the hydrate formation region 1550. Recentlydiscovering that the overall efficiency of hydrate desalination isfundamentally a function of the difference in temperature between thedissociation and formation regions; I have modified my previouslydisclosed methodologies to remove that heat of hydrate formation fromthe hydrate formation region far more quickly so as to minimize heatingof the hydrate formation region, thereby improving efficiency. Thus, tothat end, the overall movement of water through the system, includingresidual water, is controlled so as to be substantially downward and outfrom the bottom portion of the installation. The buoyancy of the hydrate1464, however, is greater than that of either the heated residual fluidor the input water-to-be-treated 1450, and therefore it will rise andconcentrate in the dissociation region 1512 as noted above. Thus,according to the desalination method of the present invention, waterflow through the apparatus is controlled such that the hydrate separatesfrom the residual fluid at or near the point of hydrate formation,thereby remaining cooler and more stable longer than previously andincreasing efficiency of the system.

The brines or residual fluids 1460, which may be mixed somewhat inturbid regions of the hydrate formation section, are drawn downward andextracted from the fractionation column 1510, out through column 1540,by pump 1466 or by natural flow driven by density within the system as awhole. Although some small amount of warm residual fluid will rise, itwill mix with new input water and cool and, because it will be moresaline than the newly input water-to-be-treated, it will sink back intothe hydrate formation region where it will be subject to further hydrateformation. Thus, per the invention, virtually all the water that isresidual from the hydrate formation process exits the fractionationcolumn 1510 apparatus at its base through column 1540, from which it canbe pumped or allowed to flow for further treatment and/or disposal. Thisis in distinct, inventive contrast to my previous methodologiesdescribed above and claimed in my referenced co-pending applications.

By locating the output of the input water pipe 1530 above the hydrateformation region 1550, hydrate formed in the hydrate formation regiontends to be additionally cooled as it rises in the apparatus by virtueof the incoming water-to-be-treated passing over it and carrying awayliberated heat of formation. (In view of this heat-flushing function ofthe water-to-be-treated, generally more seawater may be passed through(i.e., out of) the apparatus for a given volume of fresh water to beextracted than in the above-described embodiments.) This has thebeneficial effect of increasing the stability of the hydrate at lowerpressures and reducing the depth at which the hydrate will begin todissociate, which permits more efficient management of the distributionof thermal energy within the apparatus as a whole.

In cases where the residual fluid resulting from formation of thehydrate is cooler than the input water-to-be-treated prior to anycooling, it may be advantageous to return the residual fluid to heatexchanger 1508 for heat exchange with intake water-to-be-treated, in thedissociation region 1512, or, alternatively, to a region where external,artificial refrigeration is carried out as disclosed in the '422application and described above. In such cases, more hydrate would berecovered from a given volume of input water because the water would bepassed through the apparatus more than once, and less energy would berequired to cool the recycled water to the necessary temperature forformation of hydrate than in the case of cooling of warmer, untreatedwater.

The control valve and pump assembly 1466 is used to regulate water flowthrough the apparatus and permits close control over the requisite,generally higher (as compared to my earlier embodiments) extraction rateof residual water from the base of column 1510. The control valve andpump assembly 1466 permits the residual fluid to be directed either backto the input water for recycling through another hydrate formation cycleor to disposal. Because a small amount of hydrate in the form of lesshydrodynamic and/or less buoyant flakes may be removed from thefractionation column in the residual water, provision is made forcapture of the hydrate-forming gas released when dissociation of thoseflakes takes place using gas capture apparatus similar to that used inthe main hydrate dissociation region 1512. Gas released at the top ofthe brine pipe 1520 is returned to the main gas capture and processingsystem as soon as possible.

Because the flow of the input water-to-be-treated 1450 is generallydownward in the column 1510, it may retard slightly the upward movementof the hydrate 1464. However, as the hydrate rises into the relativelymore static input water located above the input water injection point,its rate of rise increases. Care must be taken to ensure that thebuoyant ascent rate of the hydrate is greater than the downward flowrate of both the input water 1450 as well as the general downwardmovement of residual water out through the base of the column 1510. Thedensity of the hydrate, and hence its buoyancy, is little affected byits temperature or the temperature of the surrounding water. Hence,although the temperature of each watermass affects the density of thewater, the solid hydrate coefficient of expansion ensures that over thesmall range of temperature variation encountered, the change of volume(and hence density) of the hydrate is insignificant.

In a preferred embodiment of the present invention, a valve mechanism isprovided that can completely close the shaft 1510 to rising hydrate1464. For example, as illustrated in FIG. 25, valve 1566 is providedwhich is a sphincter or circular-section, camera-shutter type valve inwhich a series of curved metal plates advance obliquely in a rotationalmanner, thereby closing an orifice symmetrically. Other types of valvesalso may be used. Valve 1566 is located immediately below the depth 1505at which hydrate ceases to be stable in any particular apparatus andserves a number of purposes. For example, closing of this valve uponsystem start-up allows a large amount of hydrate produced in the hydrateformation section 1550 to concentrate over a period of time. Then, whenthe valve is opened, large volumes of hydrate are released into thehydrate dissociation region 1512 so that the heat-sink potentialprovided thereby is large from the beginning of any production run. Thisfacilitates start-up and reduces the time during which start-up can takeplace. Additionally, where there is danger of gas escape within thesystem due to natural disaster such as a major earthquake, fire, or gasmain breaks, such that it may be necessary to shut down the systemrapidly without allowing gas to be passed back into the supply system orto be stored safely on the surface, hydrocarbon or other potentiallycombustible gases can be safely stored as hydrate within the shaft ofthe apparatus since hydrate is a safe way of storing and compressing gaswhich, in its gas phase, is otherwise potentially explosive orcombustible.

The presence of valve 1566 greatly enhances safety. For example, in thecase of a minor emergency, where shutdown could be made in a controlledmanner over a period of time, the valve would be closed and all the freegas in the system would be pumped to the hydrate formation region wherehydrate would form. Thus, the gas would be safely stored as hydrate inthe upper part of the shaft. When released following passage of theemergency, system restart would be very rapid because of the largeheat-sink that would be provided by the released hydrate within thehydrate dissociation region 1512.

Where the emergency was more severe and little shut-down time wasallowed, all the gas present in hydrate that was rising in the columncould be trapped within the column by quickly closing the cut-off valve1566. At any moment of operation of a hydrate fractionation desalinationapparatus, most of the gas in the system will be present in the form ofhydrate. Other gas in the system could either be stored within the gasrecovery and processing and injection systems or vented by flaring toprotect the system as a whole. The hydrate-forming gas stored in thehydrate would be essentially inert, but if pressures in this storedhydrate region did drop for any reason and result in conditions ofhydrate instability, hydrate dissociation would be slow because of the“negative feedback effect” resulting when hydrate dissociation causescooling to the point that the water released may freeze to water-ice,thereby increasing conditions of hydrate stability, and the produced gaswould dissipate or be flared. Or course, the desalination apparatus inaccordance with the present invention could be constructed and operatedwithout cut-off valve 1566 if so desired.

Provision is made for the removal of slight remaining amounts ofresidual fluid, present as interstitial water, near the upper part ofthe column 1510 at interstitial water extraction point 1517, in a mannersimilar to that described in the '422 application and above. This allowsa portion of the cold input water-to-be-treated to rise interstitiallyin the hydrate slurry and provide additional cooling to the hydrate,thus allowing the hydrate to remain stable at lower pressures. Fresherinterstitial water that flows down from the dissociation region 1512displaces the interstitial, residual water and remains above theinterstitial water-to-be-treated or interstitial residual water becauseof their density contrast and facilitates better, more efficientconversion of hydrate and its residual water matrix to fresh water.

Where this extracted water 1517 is lower in temperature than uncooledinput water-to-be-treated, it may be recycled through the apparatus,thereby reducing the total amount of water-to-be-treated by producing agreater volume of fresh water from a given volume ofwater-to-be-treated. The extraction of residual water is accomplished bydrawing it off so that the hydrate continues to rise, albeit at a slowerrate, above a “water trap” 1564 (a physical baffle), which provides adownward path for water being drawn off within the apparatus but downwhich the buoyant hydrate will not pass because of its positive buoyancyand upward progress.

In addition, lower salinity “wash” water taken from the dissociationregion is injected into the column at injection point 1518 in order todilute the slight amounts of interstitial residual water. This injectedwater, in addition to or in place of water descending from thedissociation region 1512, will lower the total dissolved solids in theresulting fresh water. The injected water may be either fresh or “grey”water, that is, water having a salinity between fresh water and water ofthe salinity of normal seawater, and may be derived from another hydratefractionation apparatus or from an external source.

Injection of the hydrate-forming gas 1502, water-to-be-treated 1450, and“wash” water 1518 may be continuous or varied so as to achieve the bestdesired results. In addition, the extraction of residual water may beeither continuous or varied so as to achieve the desired results.Baffles (not shown) may be provided within the hydrate-forming region tocontrol turbulence and direct water flow and constrain hydrate movementso as to achieve design goals for the most efficient formation ofhydrate and separation by fractionation of the different density liquidsand the hydrate.

The water-to-be-treated 1450 is carried downward in the shaft liner, orin a pipe within the shaft, or in a separate passage 1530 as shown inFIG. 25. A connection between the shafts is made at the base of theshaft 1540, which may be inclined or horizontal as determined by finaldesign to achieve operational objectives. The input water-to-be-treated1450 may be passed through a heat exchanger 1508 in the dissociationsection 1512. Alternatively, if the benefit of the cooling effect ofhydrate dissociation is not required by the design of an installation,the input water can be passed solely through an artificial refrigerationapparatus 1509, if necessary.

The exact, relative placement of the input water injection point andhydrate-forming gas injection point in the hydrate formation region isdetermined by the particular requirements for each installation. Forinstance, where a particular percentage of hydrate is desirable to formfor the particular water and apparatus, hydrate can be formed at variouslevels, with different percentages of water being extracted by hydrateformation from the water-to-be-treated at the different levels. Thisserves to distribute the production of heat over a greater verticaldistance in the hydrate formation region 1550 and promotes turbulence inthe region as a whole. In addition, hydrate-forming gas and water inputscan be interleveled, with some water-to-be-treated being input below atleast one of the hydrate-forming gas injection points (not shown inFIGS. 1 and 2). This may be desirable where some local cooling isrequired, where certain percentages of hydrate formation are required,or where turbulence is to be further promoted. Provision is allowed forsome installations to have multiple water and gas injection points,although the overall flow of residual water remains essentially downwardand some of the cold water-to-be-treated may rise to balance themovement of water in the system as a whole.

To provide for extra cooling of input water-to-be-treated if necessary,further refrigeration apparatus 1509 is applied, if necessary, on thewater inlet 1530, as illustrated in FIG. 25. The exact placement of therefrigeration apparatus and its overall length in the column is notfixed, but will be adjusted to achieve maximum desired effect for theparticular requirements of each installation.

In the embodiment shown in FIG. 25, fresh water is drawn off near thetop of the hydrate dissociation region, at fresh water extraction point1572. Furthermore, provision is made for extraction of small amounts ofwater at tap 1574, preferably located near a restricted area of watercollection near the gas extraction point 1576. The water immediatelybelow the gas interface may be enriched in particulatematerial—particularly very small material such as bacterial or viralcontamination concentrated by continuous gas flow through the producedfresh water—and its separation aids water quality and purity. (Suchprovision for the removal of finely concentrated material of this typealso may be provided in other hydrate fractionation desalination andwater purification apparatus, as described above.) Provision is alsomade for extraction of water near the base of the dissociation andseparation section 1578, as it may be necessary to remove “grey” orproduced fresh water which has too high a salinity to use as highquality product water but which may have use in agricultural or otherapplications or as wash water 158 in the apparatus.

In another embodiment of the present invention, illustrated in FIG. 26,artificial refrigeration is provided in the hydrate formation region sothat a higher proportion of fresh water can be extracted from a givenvolume of water-to-be-treated by formation of hydrate than can beaccomplished without artificial cooling. For example, as illustrated inFIG. 26, artificial refrigeration elements 1513 are provided in thehydrate formation region 1550 of column 15210. The artificialrefrigeration elements 1513 can include, for example, means for standardrefrigeration techniques such as heat transfer from one region toanother using another fluid in a closed system of pipes and radiators asthe heat exchange medium. In another embodiment, direct cooling of therefrigeration apparatus in the hydrate formation section 1513 can beaccomplished by thermoelectric techniques or other techniques, wheresome or all of the residual water 140 is used as the heat sink in ornear the hydrate formation section. The effect of cooling the hydrateforming reaction results in the formation of more hydrate per unitvolume of input water-to-be-treated than if the hydrate forming regionwere not artificially cooled. This enhanced formation of hydrate, whichresults in higher volume percentages of fresh water production, can beimplemented in embodiments of both naturally and artificiallypressurized apparatus.

According to one approach to providing such cooling, used either aloneor to supplement standard refrigeration techniques, dissociation of thehydrate (which has a demand for heat energy about equal to the heatproduced by the formation of all the hydrate) can provide most, if notall, of the heat-sink capacity for the refrigeration apparatus 1513 inthe hydrate formation region. Although implementing an artificialcooling system adds to overall cost of a hydrate fractionationdesalination apparatus, the cooling it provides will allow much higherproportions of fresh water to be recovered from a given quantity oftreated water.

On the other hand, operation with little or no cooling in the hydrateformation section at all may be possible or allowable where energy costsare sufficiently low or discounted as an operating cost; where the costsof pumping larger amounts of water-to-be-treated is not prohibitive; orwhere only slightly enhanced salinity residual water produced by theformation of hydrate is either desired or required, e.g., where thesurrounding marine ecosystem must be minimally disturbed. The treatedwater must then be cooled in situ in the hydrate formation region 1550because the indefinite heat-sink of the ocean is not available (unlesslarge quantities of cold, deep water can be pumped through theapparatus).

In the embodiment illustrated in FIG. 26, the water-to-be-treated iscooled in the hydrate formation region 1550 as the hydrate is formed.Injection of the water-to-be-treated 1450 (as in the embodimentillustrated in FIG. 25) is above the injection point for thehydrate-forming gas 1502, and the residual fluids are extracted at thebase of the column 1510 as described in connection with FIG. 25. Hydrate1464 separates from the residual fluid at or substantially at the pointof formation and rises through the input water-to-be-treated, collectingand concentrating in the dissociation region.

As with the embodiment of FIG. 25, cooling of any remaining residualfluid along with the hydrate strongly affects the density of theresidual fluid, which, instead of being partially slightly positivelybuoyant, is negatively buoyant and therefore sinks to the bottom of thesystem. The density of the residual fluid increases as hydrate isincreasingly formed because the residual fluid being produced isincreasingly saline while the temperature remains within a requiredrange, e.g., about 3° C. to 7° C. The increased salinity ofwater-to-be-treated, where it is chilled residual water, will have someimpact upon decreasing the field of hydrate stability (an inhibitor orantifreeze effect), but this effect will not significantly affect theformation of hydrate. The range of temperatures extant in the hydrateformation region is kept well within the pressure-temperature range ofhydrate formation and stability. In addition, depending on the originaltemperature of the water-to-be-treated, the final residual water islikely to be cooler than the original temperature of the inputwater-to-be-treated prior to its being taken from the sea, and it can beused to pre-cool raw input water that is at the ambient temperature ofthe source water (likely to be quite elevated in low latitude desertcountries, for instance).

In the embodiment illustrated in FIG. 26, the dissociation section isdesigned to facilitate the formation of variable amounts of hydrate withcooling provided by (but not necessarily entirely from) heat transferbetween the hydrate formation region and the heat-sink of thedissociation section 1512. The water-to-be-treated is kept cool byrefrigeration so that the temperature range in the hydrate formationsection remains suitable for the spontaneous formation of hydrate.Located above the hydrate formation section, and similar to theembodiment in FIG. 25, is a hydrate concentration section 1560, whereany slight amount of remaining residual water of elevated salinity isdisplaced by fresher water descending from the dissociation andseparation section 1570, and possibly from external sources 1518.

The cooling potential of the heat exchange provided by the dissociationof hydrate in the hydrate dissociation region 1512 will exceed thecooling requirements of the input water alone when high proportions ofwater are extracted from given volumes of treated water in the hydrateformation section 1550 (by forming large proportions of hydrate). Thisadditional cooling potential can be taken advantage of by cyclingcoolant fluid through heat exchangers 1508 in the dissociation regionand then to the refrigeration apparatus 1513 in the hydrate formationsection 1550 via shaft 1581. This use of the heat-sink provided by thedissociation reaction to provide substantial cooling potential to therefrigeration system in the hydrate formation section dramatically cutsthe cost of the artificial refrigeration. If the heat removed in theresidual brines and interstitial waters, as per the previously describedembodiments, is a significant proportion of the overall heat in thesystem with respect to the cooling demand (cooling inputwater-to-be-treated plus cooling the hydrate formation region that isbeing heated by formation of hydrate), it is possible that relativelylittle or no artificial cooling 1507, 1509 may be required, even whererelatively high proportions of hydrate are formed in water undergoingtreatment.

Where the lower part of the hydrate fractionation desalination column isdrilled, special liners carry all passages and communication lines.Controls and valves are designed to be replaceable using RemotelyOperated Vehicles within the flooded shaft, so that maintenance andrepairs can be effected without shut-down. Line cleaners are eitherhydraulic pressure driven or mechanical, or both, as design indicates.Where the shaft is larger, replaceable pipes can be affixed within theshaft to achieve the desired water flows. Redundancy (multiple orfall-back systems) of all piping and controls will provide the greatestpotential for continuous production of water, even during majormaintenance periods. Provision is made for spot heating to melt hydrateencrustations as necessary. All released hydrate rises buoyantly andcontributes to the production of fresh water.

For both naturally and artificially chilled embodiments, closemonitoring of temperature and salinity—along with other physicalparameters required to be known for optimal control of the process ofhydrate formation, separation, and concentration—will be maintained sothat input of water-to-be-treated and extraction of residual water canbe controlled to keep the thermal and salinity gradients within theembodiments in the most suitable ranges.

Installation of either of the herein described embodiments, combinedwith a general downward flow of water-to-be-treated and, in particular,the residual fluids, results in improved performance over theembodiments of the '422 application and simpler construction. Inaccordance with the present invention, in contrast to the '422application, enhanced salinity residual water produced as a byproduct ofhydrate formation flows downward instead of upward, with the heat offormation largely being carried away with it and the hydrate beingseparated from it at or near the point of formation. The bulk of theresidual water is removed from the main hydrate fractionation column orshaft at a level near or below the hydrate-formation region near thebase of the apparatus. In this embodiment, the bulk of thewater-to-be-treated is carried downward in the shaft liner or in a pipewithin the shaft, and the return flow of saline residual water is eithercarried upward in a separate pipe or specially designed watercoursewithin the shaft liner or in a separate lined pipe drilled to the baseof the desalination fractionation column.

Many variations of lay-out of this embodiment are possible. The preciselay-out of the input and residual water streams are engineered for eachsituation to achieve best results. For example, as illustrated in FIG.27, shafts are drilled and lined appropriately for operation of adownward flow hydrate fractionation desalination column. This lay-outallows close spacing of a number of desalination apparatus which allfeed into a single residual fluid collection column, which is reservedfor saline water return 1577 to the ocean or to the surface for disposalor further non-desalination utilization.

The amount of hydrate which exits the fractionation shaft or column intothe residual return shaft is likely to be small in proportion to theamount of input and fresh water produced. However, provision forrecovering hydrate-forming gas from the residual water to be returned1516 is made to comply with good business and environmental practice.The residual water stream to be returned to the ocean may be furtherdegassed. The return shaft can be narrower in diameter and lined moresimply, for instance with little or no insulation, which will result ina low-cost connection to the base of the desalination columns.

In accordance with the embodiment of the present invention asillustrated in FIG. 27, grouping a number of desalination fractionationshafts 1622 that feed their residual saline water into a single waterreturn shaft 1612 will economize on the number of holes that must besunk, and thus the cost of the installation. Four desalination shafts1622 are shown in FIG. 27 for illustration only; the actual number ofshafts will depend on engineering objectives and other concerns, but maybe either more or less than the number herein illustrated. In yetanother embodiment illustrated in FIG. 28, a symmetrical distribution ofdesalination fractionation columns (water-to-be-treated shafts) 1614 andresidual water return shafts 1624 results in a very efficient use ofsurface area for an installation based on a number of shafts/holesrather than a single or a very small number of much larger holes, whichhave a greater cross-sectional efficiency for allowing hydrate tobuoyantly rise. The dissociation chambers which are near or at thesurface may have groups of shafts feeding hydrate to them, or they maybe offset laterally, as dictated by the design of each installation soas to take into consideration construction, operational, andenvironmental concerns.

Where shafts are carrying water of different density, the level at whichwater is fed into or removed from each shaft may be implemented in sucha way as to result in balanced pressures within the system as a whole,even for the generation of a controlled driving force that will pullintake water into the system and expel it naturally. For example, if theheight of the water take-off point from residual water column is suchthat the weight of more dense water in the brine shaft 1520 is less thanthe weight of the sum of different waters and hydrate in the mainhydrate fractionation shaft or column 1520, then the natural flow ofwater will be through the system as a whole, which may remove therequirement to expend energy pumping water through the system. Further,if the system is located so that its top is below sea level, water willflow naturally into the system as well as through the system. Further,where the residual brines are piped to the ocean, ideally at a greaterdepth than the input water, its natural density will cause it to flownaturally from the pipe into the sea, where it can mix and disperse. Ifthese aforementioned conditions are met, then little artificial pumpingof water into, through, or out of the system and back to the sea may berequired. Because of the density changes within the system, and itsgenerally unobstructed nature, the design of each system will beoptimized for the local conditions, especially with respect to thetemperature of the watermass to be treated.

The economics of construction of a desalination facility may favor thedrilling of many holes, as these can be drilled while the hole is wetand lined during and/or after drilling, with wireline methods used forthe extraction of core or crushed rock extraction where surface drillingmuds and ponds are not a significant issue. Sunk shafts, on the otherhand, involve construction under essentially dry conditions, whichrequires very high pumping, debris removal, and manpower costs incomparison to drilling. A symmetrical distribution of replicate shafts(as shown in FIG. 28) will allow not only for use of the same drillingtechnique for each of the different types of hole but for massproduction of the required liners and control apparatus, which can bethe same in a variety of different sites. In addition, a symmetricaldistribution allows a group of desalination fractionation shafts 1614 toefficiently share input and residual shafts, and for a facilityinitially composed of a minimum number of water-to-be-treated inputshafts 1634 and residual saline residual water shafts 1624 to beexpanded without the need to cease production of that part of thefacility already in production. Because it should be possible in manycases to put the control building, dissociation chambers, and gashandling and processing apparatus directly above the various shafts, anefficient use of area for desalination installations of this type (e.g.,a small footprint) is possible.

A hydrate fractionation installation comprised of a number of shafts hasother advantages where a large proportion of the energy consumed is usedto cool intake water from very hot water sources, such as in restrictedseas adjacent to desert areas. When this cooling charge is relativelyhigh, producing as high a proportion of fresh water from any particularvolume of water-to-be-treated will have the effect of lowering theoverall cost of produced fresh water. Where the proportion of hydratethat can be formed in water-to-be-treated is desired to be less than themaximum proportion of fresh water extraction desired because of thephysical character of the fluid, for instance, the residual water fromone column can be cooled and input into another for a further cycle ofrelatively high proportion hydrate formation. Passing a particularvolume of water through a process that extracts the maximum amount offresh water from water-to-be-treated in the form of hydrate more thanonce will have the effect of recovering extremely high proportions offresh water from any given volume of water.

In another embodiment (not shown), cooling and/or refrigerationapparatus can be affixed to pressurized hydrate fractionationdesalination apparatus which may be employed in artificially pressurizedinstallations. In such an embodiment, heat is removed from thehydrate-formation chamber or chambers so that the proportion of waterthat can be extracted from a particular volume of water-to-be-treated byhydrate formation is not limited by the heat produced by the exothermichydrate formation reaction. This is beneficial to the cost of freshwater production because all water used must be fully pressurizedartificially; thus more fresh water can be recovered from a given volumeof water-to-be-treated. The elements of the cooling apparatus canconsist of only heat exchangers where the ambient temperature of thesurrounding environment is suitably cold, but may also consist of arefrigerator unit so that very cold fluid circulation provides forefficient cooling regardless of the ambient temperature in theenvironment surrounding the pressurized desalination apparatus. Inaddition, the heat sink of the hydrate dissociation reaction may be usedto cool input water-to-be-treated. The refrigerator coils can be placedentirely on the outside of the pressurized apparatus but communicatingwith heat exchanger vanes within the vessel, or the refrigerator coilscan extend into the vessel, as determined by individual installationdesign.

Where the ambient temperature is cold, the heat extracted from thehydrate formation stage of the desalination and water purificationprocess may be used for heating habitations or for other purposes.

The difference in volume between the volume of water-to-be-treated andthe volume of residual water is the volume of water extracted by theprocess of formation of hydrate. Close monitoring of the water andhydrate volumes is required because the actual act of desalination takesplace when hydrate is formed by extracting water molecules from theseawater. One cubic meter of hydrate contains on the order of 0.87 cubicmeters of water; thus, as the volume of hydrate in the apparatus varies,it will displace variable amounts of water. Long operating runs duringwhich a small range of variation in all components within the system aremaintained is thus' an important operational objective.

In considering the costs of building and operating a desalinationinstallation of the hydrate fractionation type, the overall pressuresrequired control the size and strength of the apparatus to a largeextent. Where an apparatus must reach to deep depths to provide for along column of water to provide pressure in the hydrate formationregion, or where the apparatus must be strong enough to withstand highinternal pressures, its costs will be relatively high. Thus, it isimportant to consider means by which pressure in the apparatus as awhole can be lowered in order to lower costs.

Hydrate formation depths vary depending mainly on temperature andpressure, which control the formation and stability of gas hydrates. Thechemistry of the water in which hydrate may form may have a lesseraffect on modifying the field of hydrate stability.

Different gases and mixtures of gases have substantially differentranges of stability and these are of particular interest wherehydrate-forming-gas mixtures suitable for forming hydrate from seawateror polluted water can be used to extract water molecules into thehydrate and restore them to liquid (fresh water) upon concentration anddissociation. For example, where ethane, propane, butane, or some othergas that in the mix lowers the pressure and/or raises the temperature atwhich hydrate will form may be mixed with methane in differentproportions, the pressure-depth of hydrate stability at varioustemperatures is significantly shallower than the pressure-depth formethane alone. This has important ramifications where operationalvariables of a hydrate fractionation desalination system are to bedetermined for the most efficient operation.

For instance, one of the major cost factors of projected shaft andcombined shaft and artificial pressurized desalination fractionationinstallations is the length of the shaft that is necessary to providerequired design pressures. It is anticipated, for instance, that up toapproximately 5% of propane in methane in the hydrate-forming-gas is anachievable operational gas mix, although a higher proportion is alsopossible. Use of this propane mix in methane indicates that the totalpressure required is about one-quarter of that for methane alone, withvirtually any propane mixed with methane halving or more than halvingthe minimum total pressure required to produce gas hydrate. Where theinstallations can be operated at lower pressures, economies can beachieved in both the capital cost of the building of a hydratefractionation desalination installation and its operating costs. (One ormore of ethane, butane, or other gases can also be used in addition toor instead of propane, with similar advantage.)

Finally, as noted above, it may be preferable to dissolve an amount ofhydrate-forming gas in at least part of the water to be desalinated orpurified prior to it being injected into the desalination fractionationapparatus and being mixed with hydrate-forming gas or gases under theconditions conducive to formation of hydrate. FIG. 29 illustrates aportion of a desalination fractionation column in accordance with thataspect of the present invention, which provides for saturating thewater-to-be-treated with hydrate-forming gas or gas mixtures prior tohydrate formation in a land-based installation.

As illustrated in FIG. 29, a hydrate fractionation desalination andwater treatment installation made on land uses a shaft or large diameterdrill hole 1650 to reproduce the pressure found naturally at sea. Inthis embodiment, the hydrate-forming-gas line 1654 and the inputwater-to-be-treated pipe/conduit 1664 are separate from the main shaftor hole 1650 where the hydrate 1670, once formed, can rise buoyantly.The hydrate-forming-gas line 1654 and water-to-be-treated pipe/conduit1664 are shown separately here for clarity, but each, or both, may becarried within the liner to a single shaft rather than in separatecourses. Hydrate-forming-gas is taken from the gas pipe 1654 by acontrolled regulator 1658 and supplied to a mixing chamber 1660 where itis mixed with the input water in a turbulent manner so that a maximumamount of the hydrate-forming-gas is absorbed by thewater-to-be-treated. The controlled regulator 1658 meters thehydrate-forming-gas so that desired amounts of that gas can be dissolvedin the water-to-be-treated present in the mixing chamber 1660.Alternatively, a separate gas line (not shown) may be used to introducea gas or gas mixture different from that of the hydrate-forming gas. Themixing chamber 1660 is preferably located above the pressure depth(illustrated by line 1662) at which hydrate will spontaneously form asdetermined by the water within the system and the temperature of thewater-to-be-treated.

This gas mixing pre-treatment process can be continuous, in which casealmost all of the water is pre-treated, or intermittent, in which casenot all of the water is pre-treated. The precise method will vary foreach site and on changing environmental conditions, such as changes inthe temperature of the water-to-be-treated.

After the gas mixing pretreatment process, the gas pre-treated water1665 is directed to hydrate formation region 1667 of the column 1650.The gas pre-treated water 1665 is mixed with the hydrate-forming-gasfrom gas pipe 1654 to spontaneously form hydrate 1670, and pure water isobtained from that hydrate in accordance with the methods disclosedabove.

It will be appreciated that the embodiments disclosed herein areillustrative, and that numerous variations to and departures from thespecific embodiments disclosed herein can be made while remaining withinthe spirit of the invention. All such modifications to and departuresfrom the disclosed embodiments are deemed to be within the scope of thefollowing claims.

1-12. (canceled)
 13. An installation for desalinating or purifyingsaline or otherwise polluted input water, said installation comprising:a desalination fractionation installation having a hydrate formationregion disposed at a lower portion of said installation and a hydratedissociation region disposed at an upper portion of said installation; amixing chamber; an input water conduit which is arranged to provideinput water to said mixing chamber and to said hydrate formation region;a gas supply conduit which is arranged to provide hydrate forming gas tosaid mixing chamber and to said hydrate formation region; and whereinhydrate forming gas is dissolved into at least a portion of the inputwater in said mixing chamber prior to being input into said hydrateformation region.
 14. The installation as set forth in claim 13, whereinsaid input water conduit which is arranged to provide input water tosaid mixing chamber and to said hydrate formation region constitutes afirst input water conduit arranged to provide input water to said mixingchamber and a second input water conduit arranged to provide input waterto said hydrate formation region.
 15. The installation as set forth inclaim 13, wherein said gas supply conduit which is arranged to providehydrate forming gas to said mixing chamber and to said hydrate formationregion constitutes a first gas supply conduit arranged to providehydrate forming gas to said mixing chamber and a second gas supplyconduit arranged to provide hydrate forming gas to said hydrateformation region.